The Sainsbury's supermarket Microwave Oven....

The wife called me at work...

"Mum's microwave has died, can you have a look?"

Yeah .... why not ...

It's a cheap and cheerful microwave oven from Sainsbury's....

... everything works, except it doesn't get hot.

First things first ... look at the warning in the top right hand corner of my website, the bit in red.

Every microwave ever made, carries this warning... I must insist you follow it's warning.. leave it to the professionals... m'kay?











Right, being the professional, I unscrew the cover to reveal the inner workings of the oven...











Here's an annotated picture...












First thing to check is for safety. The high voltage capacitor can store ~2,500 volts , let's make sure it's safe before going anywhere near it. There's enough charge in there to floor you, or kill you. I check it's dead with a high voltage probe.... It's not charged up ... good.

It's then time to check the high voltage fuse, which turns out to be open circuit :)

Now it's not an ordinary 1.25" fuse. It's a special high voltage fuse. A like-for-like replacement is ordered and fitted.

This poor fuse suffers a lot of stress in it's life, and most seem to fail through old age, rather than protecting a failed circuit.




Here's a diagram of the all important high voltage section I've pinched from a Daewoo manual. It neatly shows the simplicity of the circuit.
















... and here's the obligatory video....

"Solder creep" and an unpleasant smell...

Just been sat in the workshop doing a few little jobs, and there's a funny small of something getting too warm. I've noticed it on and off for a couple of days, thought it was the wood burning stove in the other room yesterday, but tonight, it's definitely coming from round the computer area.... immediate investigation required.

After a couple of mins I found it.

There's an extension block connected to the output from a UPS. It's warm. Damn warm....


Further investigation shows the neutral has been overheating. I've disconnected it here. The wire has been soldered! Never, ever solder a mains cable (or any stranded cable for that matter) before fitting it into a screw type fitting. The solder "creeps" up the cable, and leaves a resistive loose connection behind.

This lead was carrying less than 500W ... so nowhere near maximum load.

I wonder how many more of this brand I have?

Not many I hope. This one's for the bin...

CCT811 Video modulator massive failure.

Remember my warnings and concern about the CCT811 video modulator?

This is the offending model in question. Apparently it's also being sold under various guises, one model quoted to me is the RF9000. If it looks like this, I'd be seriously concerned....

I wrote about it here. Please take a moment to read this, if you haven't seen it before.

Well, settling down for an afternoon of vintage TV fun, I switched the video rack on....

Crack, crack, crack, bang! Uh-oh...

"It's bound to be the modulator" I thought.


I was right. It had opened the fuse I'd retro-fitted. (If you have one of these awful modulators, time to stop using it!) Damned glad I fitted one... lord knows what would have happened if it had tried to short out the mains without the protection of a fuse... Fire would have been a REAL possibility.

I opened it up and expected to find a mass of blackened bits, but no.  Now I've giving up on this I thought... but it's so useful!

It looks as though the insulation had failed on the transformer, and destroyed the semi-conductors in the drive-side of the supply.

I thought I'd remove the ghastly existing "switched-mode" blocking oscillator, and replace it with something ...

1) Electrically safe.
2) Reliable.

OK. First things first. To find out if it still functions.

I removed the transformer, and connected the workshop power supply between the end of the rectifier diode, and ground....






It seems to want about 6.5 VDC to operate, and has even remembered the settings it had before the power supply failed... good.

It's drawing about 165mA at that voltage, so a supply is not too challenging!

I decided to have a look in the drawer of redundant wall-warts to see if I could find anything suitable, and I spy one of my favourites, an old Nokia phone charger!

Now I always pick these up from boot-sales for a few pence, and are very useful. This one is rated at 3.7 Volts, at 350mA...

"But Andy, you said the modulator needs 6.5 volts to work" ... and so it does, but this particular Nokia charger is unregulated. Off-load it makes about 10 Volts.... Will it make our 6.5 volts at 165mA?

I solder the white +VE lead to the anode of D107, and the black lead to the far right hole (viewed from the rear of the modulator) left behind when I removed the transformer...










... and switch on....

Eureka! It works.










So it's now electrically safe, and I can sit back, and watch some "proper" TV....

QED Mains Interference Suppressor 6/4C

This arrived in the workshop, smelling foul.

It's a mains filter. A simple LC network in line with a mains plug and socket. There's an X-class capacitor on the output side, which had obviously had enough. The stench was horrible. On with some gloves, and clean the thing up!

A new capacitor was soldered in, and the filter reassembled.

These caps have a hard life. It's difficult to date them, as the same Rifa capacitor is still available today.

Apologies for the appalling photo. I've got a new SLR camera on the way, which will hopefully improve matters. 

Video rack video modulator repair and warning! Model CCT811

Remember the video rack I built a while ago? It's details are here.

Well, it's developed a fault. The UHF modulator has been giving very poor pictures after a few minutes of operation. I decided to take it out, and repair it.

IF YOU HAVE ONE OF THESE UNITS, I WOULD HAVE SERIOUS RESERVATIONS ABOUT IT'S ELECTRICAL SAFETY. 

I purchased it ages ago from eBay, and it came fitted with an unfused euro plug, which I cut off, and fitted a standard UK Plug, and fused it at 1 amp. I'm glad I did... read on ...

It's a useful thing. Is stable (when working properly), is adjustable throughout European VHF allocations, as well as UHF, and has switchable FM sound between 5.5 MHz and 6 MHz sound sub-carriers....

Disassembling the unit reveals a few horrors....

Check the mains input to the diminutive switched-mode supply ..... No fuse is evident!

What you can't see by this picture is the bottom of the case, the black bit. It's metal. There's no earth, which would be fine if the thing met Double Insulated (Class II) standards, which I doubt it does. You can read about classes here.

There's also zero filtering on the mains input, so any noise from the switched mode makes it's merry way back down the mains lead and out onto our mains, spoiling our radio reception (and, ironically, our TV reception too!) if it radiates (which it will).

Grim. Glad I fitted a fused plug ....

Examination of the power supply shows it to be nothing more than a simple blocking oscillator.

There's a simple zener on the output attempting to provide a little regulation. You can see where the board is a little discoloured, as it's been running warm. There are two capacitors in the primary, the mains smoothing capacitor (4.7uF 400V) proved to be very low in capacity. I fitted a 10uF , as I had one to hand. I also changed the smaller cap (10uF 50v) whilst I was there.

 A quick check shows the unit to be working again.... but what to do about the safety issues?

Mounting it back in the rack, I've fitted an in line filter and fuse (100mA). I'm not so concerned about earthing the case, as the modulator is inaccessible when the rack is assembled, that doesn't mean you should be though!

Pity really ... "for a ha'p'orth of tar" an otherwise good unit is spoiled.... and possibly electrically unsafe and a fire risk. It carries a CE mark, which I've no doubt it doesn't deserve.

 This gives you some idea of the size of that supply transformer, that's a 1p piece!

The guilty parties. Caps. As usual!

Arduino Mains Monitor with SIM900A GSM messaging.

Picture the scene... You're away on holiday, sunning yourself in Spain (or if you're Spanish, enjoying the damp weather in the UK !) ... and thousands of miles away, some spurious minor electrical niggle in your house causes the RCD circuit breaker to open. You return 10 days later to find the contents of your freezer oozing their way across the kitchen floor. Not nice... You reset the breaker, and the power comes back on, showing no faults.

It happens from time to time in my house. I've tested each circuit, and every appliance to try and find the cause of the random tripping. The fault is not visible on any of my insulation readings, but, nevertheless it does happen. Sometimes not for years.... frustrating!

So what can we do about it? Fix the fault would be the easiest thing to do, but it's eluding me. How about monitoring the mains, and sending me a message, so if it does trip, I can reset the power without issue? Good plan...

Now the disclaimer:

WARNING. Do NOT try this at home. This project deals with mains voltages. A shock from the mains will hurt and can easily be fatal. Work safely. Use an RCD. Disconnect from the mains before making any adjustments. 

RESPECT IT'S AUTHORITY. 

I will not, under any circumstances, accept any liability if you decide to re-create this project for yourself.


Now that's got that out of the way...

So, we have a plan. Measure the mains voltage (and why not frequency at the same time?), and act conditionally on it's failure. We also need to get it to send us a message. We're going to need some form of uninterruptable power supply, so when the mains does go off, our micro will still be running, and we've got power enough to send the message. Here's the circuit:

I'll go through it step by step...

Power comes in via a fuse to two transformers. TR1, is a mains to 15V transformer, and is used as our power supply. It feeds a bridge rectifier, B1 and C1 and C2 are used to smooth and decouple the resultant DC. The DC is fed via D2 to a 7805 regulator, which provides the 5V for the Arduino and the SIM900  module. So that's fairly straightforward. The 7805 in this instance is a 2A part, as the SIM900A does require a fair amount of current to function.

(You could, and it would be safer to do so, build this unit using two wall warts, one, with, say at 15VDC, 3A output to replace TR1 & B1, and a second, with AC output in place of TR2. That way, all the tricky and potentially dangerous stuff with the mains is eliminated)

The rectified DC from B1 is also fed to two LM317 voltage regulators in series. The first, IC1, is configured as a 100mA constant current source, this is then used to feed the second regulator, IC2, which is configured as a conventional voltage regulator, in this case, R3 is adjusted to provide around 14V. This forms a 100mA constant current charger, with maximum voltage of 14V, which we can use to charge G1, which is an old 12V NiMH battery I happened to have kicking about. It's got plenty of capacity left, so will do nicely as our back up battery. S3 is a battery diconnect switch. Useful for resetting. (Why didn't I use an LM200, as it is capable of both current and voltage regulation in one package, instead of two LM317's? Because I didn't have one!)

So, when the mains is present, our battery is being charged. In the event of mains failure, D2 will stop conducting, and the supply will be seamlessly taken over by the battery, supplying current to the 7805 via D1. D3 prevents current flowing back into the charging circuit. The battery voltage is also sampled at the mid-point of the potential divider, R11 and R12, and fed to the Arduino A1 pin. T2, between the reference pin of IC2 and ground, is used to switch off the charging circuit momentarily, so we can measure the battery voltage, and not just the output of the charger. It's controlled by the Arduino A2 pin, configured as a digital output.

OK, so that's power and back up power sorted, so what about measuring the mains? TR2 is a small (3VA) 18V mains transformer. This feeds another bridge rectifier, B2, and a small smoothing network, formed by C7. The voltage developed across C7 is fed to a potential divider formed by R5 and R4, and the resultant voltage fed to Arduino A0. This voltage will be directly proportional to the mains input voltage on the primary of T2.... or will it? Whilst I was experimenting, I noticed the measurement wasn't linear. Adding a bit of loading, in the form of R15 to the secondary helped matters no end, although it was accurate enough over the range required for this to be left out, if required.

Also coupled to the secondary is our frequency measuring network. The AC is fed via coupling capacitor C8 to the base of a BC547, T1. This provides a 5 volt 50 Hz signal to the input of IC4A (a Schmitt trigger hex interter), this will square our pulses up. The output of IC4a is fed via a low pass filter, formed by R8 and C9 to the input of IC4B, the output of which is connected to the Arduino pin D8. We'll use this to measure the frequency of the mains. Tie all of the unused inputs of IC4 to ground.

On to the micro side of things...

The LCD is wired to the Arduino in the time-honoured fashion, except for the cathode of the backlight, which isn't connected straight to ground, but to the collector of T3. This allows us to control the backlight, using A4 of the Arduino configured as a digital output. There's a momentary switch coupled to A3, which is used as a "push to transmit" function, and a toggle switch, S2, connected to pin A5, which is used to stop the SIM900 sending messages. R9 is the LCD's contrast control.

The SIMR pin on the SIM900 module is connected to the Arduino's hardware serial Tx pin, and the SIMT pin is connected to the Rx pin.

The software.....

We're going to need something to measure our frequency. I did initially use PulseIn , but it's not that accurate, so I switched to using the most excellent FreqMeasure library, available from https://www.pjrc.com/teensy/td_libs_FreqMeasure.html . Now this has a drawback. It uses Int 1, and this is in conflict with the software serial library, which is why the SIM900 is connected to the hardware serial port of the Arduino. The issue here is the hardware port is also where the inbuilt USB interface sends/receives data, so you'll need to disconnect the SIM900 from the arduino whilst uploading the sketch, or doing any serial debugging. You can use SoftwareSerial during development, but be prepared for some unusual responses from FreqMeasure!

Ok, so the sketch.

There's a rake of variables set at the start. The variables to watch are the mains tolerances, these will need to be set for your local mains (They're currently set for UK mains spec.)

  float MainsMinV = 216.2; // This sets the lower limit for the mains voltage. Change this to suit your local voltage limit
  float MainsMaxV= 253; // Maximum voltage limit
  float BatteryMin=11.2; // Battery low limit
  float MainsMinF=49.5; // Minimum allowable mains frequency limit, change to suit local power
  float MainsMaxF=50.5; // Maximum allowable mains frequency limit.

You will also need to change line 260 :

      Serial.println("AT + CMGS = \"+44xxxxxxxxxx\"");// recipient's mobile number, in international format

If your mains is not ~240v, you will also need to change the scaling factor in the software , line 286, so the voltage reads correctly. Currently the mains voltage is scaled so 250V is equal to 5V at our micro. If, say, you're on 120V mains, and want the voltage to top out at , say 130V, then the scaling will be 130/5 = 26. It's currently set to 51, so 5V on our analogue port reads as 255V :

  ACVoltage = (sensorValue * (5.0 / 1023.0))*51;

Adjust R4 to get the mains calibration correct.

So, here's the complete sketch:
/* GSM Mains Monitor Version 1.0 Written by Andy Doswell 2015 License: The MIT License (See full license at the bottom of this file) Schematic can be found at www.andydoz.blogspot.com/ A mains monitoring device, which will send SMS status messages on the condition of the supply. */ #include <FreqMeasure.h> #include <LiquidCrystal.h> double SumFreq=0; // Cumulative frequency int CountFreq=0; // Number of time frequency has been measured double SumACVolt=0; // Cumulative ACVoltage int Count=0; // Loop counter float FrequencyOut = 50; // Default frequency value float ACVoltage =0; // instantaneous ACVoltage float DCVoltage =12; // Default DC Voltage float ACVoltageAvg = 240; //Default AC Voltage boolean SMSTransmitFlag = true; // SMSTransmit flag. This is used to prevent message duplication, and also ensures a power restored message is sent on power up boolean BatteryTransmitFlag = false; // Flag set when battery voltage drops, to prevent multiple messages being sent boolean FailFlag = false; // Used to check if the mains is OK or not String text=""; // Used in the composition of SMS message int Status = 3; // Ensures the voltage and frequency loop is completed 3 times on power up, to elliminate spikes on power up, and also after SMS message has been transmitted. Prevents false triggering. float MainsMinV = 216.2; // This sets the lower limit for the mains voltage. Change this to suit your local voltage limit float MainsMaxV= 253; // Maximum voltage limit float BatteryMin=11.2; // Battery low limit float MainsMinF=49.5; // Minimum allowable mains frequency limit, change to suit local power float MainsMaxF=50.5; // Maximum allowable mains frequency limit. char Reply[200]; // Serial buffer length int Signal; // Signal strength as reported int temp1; // 4 temporary integers, used to extract the data from the incoming serial. int temp2; int temp3; int temp4; int BER; // Bit error rate int SignaldBm; //Signal in dBm char Provider [34]; // Service Privider name LiquidCrystal lcd(12, 11, 5, 4, 3, 2); // Define where our LCD is connected. void setup() { Serial.begin(9600); // Start serial comms. delay(2000); // alow GSM module time to come up before we attempt to configure it digitalWrite (A4, HIGH); // Switch the LCD backlight on lcd.begin(20,4); lcd.clear(); lcd.setCursor (0,0); lcd.print (" GSM Mains Monitor "); lcd.setCursor(0,1); lcd.print ("(C) A.G.Doswell 2015"); lcd.setCursor (0,2); lcd.print ("GSM connecting."); Serial.print ("ATE0\r"); // Set SIM 900 echo off delay (220); GetReply (); Serial.print("AT+CMGF=1\r"); // Set Serial to text mode delay (220); GetReply(); Serial.print("AT+CNMI=1,2,0,0,0\r"); // set GSM module to send an incoming SMS message to the serial interface automatically. delay (220); GetReply(); delay (10000); // wait for GSM to connect lcd.print ("."); // and draw some dots so the user knows something's happening delay (10000); lcd.print (".."); delay (10000); lcd.print (".."); delay (10000); GetProvider (); FreqMeasure.begin(); // start the FreqMeasure library delay (1500); // Give FreqMeasure some time to settle lcd.clear(); pinMode (A2, OUTPUT); //charge control pin is A2 digitalWrite (A2, LOW); //Charger on pinMode (A4, OUTPUT); //backlight control is pin A4 pinMode (A3, INPUT); // Push to transmit digitalWrite (A3, HIGH); pinMode (A5, INPUT); // GSM Inhibit switch digitalWrite (A5,HIGH); } void lcd_display() { //Update the LCD display lcd.setCursor (0,0); lcd.print ("Mains "); if (!ACVoltageAvg) { lcd.print ("00"); } lcd.print (ACVoltageAvg); lcd.print ("VAC "); lcd.setCursor (0,1); lcd.print (FrequencyOut); lcd.print ("Hz "); lcd.setCursor (0,3); lcd.print ("Batt "); lcd.print (DCVoltage); lcd.print ("V "); lcd.setCursor (16,0); if (!FailFlag) { lcd.print (" OK"); digitalWrite (A4, HIGH);// make sure the backlight is on. } else { lcd.print ("FAIL"); digitalWrite (A4, LOW); // switch the backlight off in the event of power fail, to save a few mA, as we're now running on battery } lcd.setCursor (0,2); lcd.print(Provider); lcd.print (" "); lcd.print(SignaldBm); lcd.print("dBm "); } void GetBatteryVoltage () { // Get the current battery voltage digitalWrite (A2, HIGH); // switch the charger off delay (50); int sensorValueDC = analogRead (A1); digitalWrite (A2, LOW);// switch the charger back on DCVoltage = (sensorValueDC * (5.0 / 1023.0)) * 3; } void GetACStats () { // Create the averaged outputs for frequency and ACVoltage float frequency = FreqMeasure.countToFrequency(SumFreq/CountFreq); FrequencyOut = frequency; ACVoltageAvg = (SumACVolt/Count); if (FrequencyOut == INFINITY) { FrequencyOut = 0; ACVoltageAvg = 0; } lcd_display(); ResetStats (); } void TransmitPowerFailSMS () { // Send a power fail message if the mains is out of spec, and only if TX switch is ON. if ((!SMSTransmitFlag) && (digitalRead (A5) == HIGH)){ lcd.setCursor (0,2); lcd.print ("Sending Power fail "); text = "Mains Fail "; // Set SMS message first line. ComposeMsg (); // Set the rest of the message lcd.setCursor (0,2); lcd.print (" "); SMSTransmitFlag = true; // Set The SMS flag, so we know a fail message has been sent. ResetStats (); // Reset the frequency and voltage stats. This is to prevent our frequency picking up interference from the GSM module and failing } else { return; } } void TransmitPowerRestoredSMS () { // Send Power restored message if a fail message has been sent previously, and only if TX switch is ON. if ((SMSTransmitFlag == true) && (digitalRead (A5) == HIGH)) { lcd.setCursor (0,2); lcd.print ("Sending Power OK "); text = "Power Restored. "; // Set first line of SMS message ComposeMsg (); // Set the rest of the message lcd.setCursor (0,2); lcd.print (" "); SMSTransmitFlag = false; // set the flag to false, so we don't do this again until a power fail message has been transmitted. ResetStats (); } else { return; } } void TransmitBatteryLow () { // Send battery low message if we haven't done it before. if ((BatteryTransmitFlag == true) || (digitalRead (A5) == LOW)) { return; } else { lcd.setCursor (0,2); lcd.print ("Sending battery low "); text = "Battery Low "; ComposeMsg(); lcd.setCursor (0,2); lcd.print (" "); BatteryTransmitFlag = true; // set the battery transmit flag ResetStats (); } } void TransmitStatus() { // Send requested message lcd.setCursor (0,2); lcd.print ("Sending on request "); delay (150); text = "Current Status: "; ComposeMsg (); lcd.setCursor (0,2); lcd.print (" "); ResetStats (); } void GetReply() { // Reads serial replies from GSM module. Checks to see if "OK" is sent by GSM Module int i = 0; while (Serial.available()>0) { Reply[i] = Serial.read(); delay (10); if (Reply [i]=='K' && Reply [i-1] =='O'){ Serial.read(); Serial.read(); Reply [i+1] = '\0'; Serial.print (Reply); return; } i ++; } Reply [i] = '\0'; // End the array nicely. } void GetSignal() { // request signal strength from GSM. Serial.print ("AT+CSQ\r"); delay (150); } void GetSMS (){ // Checks to see if incoming sms is "Check", if it is , then send status message. if (Reply[51]=='C' && Reply[52]=='h' && Reply[53]=='e' && Reply[54]=='c' && Reply[55]=='k') { TransmitStatus(); } Count = 0; } void GetProvider () { // This runs once, and gets the mobile network service provider ID if (Serial.available() !=0){ GetReply(); } Serial.print ("AT+COPS?\r"); delay(150); GetReply (); int i=14; while (Reply[i] !='"') { Provider [i-14] = Reply[i]; Provider [i-13] = '\0'; i++; } } void ResetStats () { // this routine resets our voltage and frequency statistics, and avoids issues between FreqMeasure and SoftwareSerial libraries sharing an interrupt. SumACVolt = 0; SumFreq = 0; CountFreq=0; Count = 0; } void ComposeMsg() { // This composes and sends the sms message text.concat(ACVoltageAvg); text = text + "VAC. "; text.concat (FrequencyOut); text = text + "Hz. Battery: "; text.concat (DCVoltage); text = text + "V"; Serial.print("AT+CMGF=1\r");// AT command to send SMS message delay(150); Serial.println("AT + CMGS = \"+44xxxxxxxxxx\"");// recipient's mobile number, in international format delay(150); Serial.println(text);// message to send delay(150); Serial.println((char)26); // End AT command with a ^Z, ASCII code 26 delay(150); Serial.println(); delay(1500); // give module time to send SMS text =""; Status = 3; } void loop() { Count++; //increment a loop counter if (digitalRead (A3) == LOW){ // Checks the "push to transmit" button, and transmitts if it's low. TransmitStatus(); } if (FreqMeasure.available()) { // Checks frequency of mains, and adds to the sum, ready for averaging. SumFreq = SumFreq + FreqMeasure.read(); CountFreq=CountFreq+1; } int sensorValue = analogRead(A0); // Reads our scales AV voltage , and adds to the sum, ready for averaging. ACVoltage = (sensorValue * (5.0 / 1023.0))*51; SumACVolt=SumACVolt+ACVoltage; if (digitalRead (A5) == LOW) {// Checks the transmit inhibit switch. lcd.setCursor (14,3); lcd.print ("TX off"); } else{ lcd.setCursor (14,3); lcd.print (" "); } if (Count ==1) { // everytime the loop counter is 1, get the battry voltage and check the signal strength GetBatteryVoltage (); GetSignal(); } if (Count == 30 && !Status) { // is the loop counter is 30, and status counter is 0, then calculate the averages. GetACStats (); if (((ACVoltageAvg <MainsMinV) || (ACVoltageAvg >MainsMaxV)) || ((FrequencyOut <MainsMinF) || (FrequencyOut >MainsMaxF))){ //if mains is outside spec then transmit power fail message TransmitPowerFailSMS (); FailFlag = true; // sets a flag to say our mains is bad. } if (((ACVoltage >MainsMinV) && (ACVoltage <MainsMaxV)) && ((FrequencyOut >MainsMinF) && (FrequencyOut <MainsMaxF))) { //If our mains is restored, then transmit a power restored message TransmitPowerRestoredSMS(); FailFlag = false;// resets the falg to say our mains is OK } if (DCVoltage < BatteryMin) { //Battery Under Voltage? Then transmit Low Battery TransmitBatteryLow(); } else { BatteryTransmitFlag = false; } } GetReply(); // clear up anything spurious in the serial buffer. if (Reply[4]=='S' && Reply[5]=='Q'){ // this routine decodes our signal strength. temp1=Reply[8]-'0'; //convert relevant characters from array into integers temp2=Reply[9]-'0'; temp3=Reply[10]-'0'; temp4=Reply[11]-'0'; if (temp3 == -4) { // use temp3 to determine where in the array the relevent integers are (-4 is a ",") Signal= temp2+ temp1*10; // calculate signal if the first digit is a multiple of 10 BER = temp4; } else{ Signal= temp1; //calculate signal if the first digit is not a multiple of 10 BER = temp3; } if ( Signal == 99) { // if our signal is 99, IE no signal condition , the return a signal of -1 Signal = -1; } SignaldBm = Signal*2 -113; // calculate dBm for geeks like me. } if (Reply[4]=='M' && Reply[5] =='T') {// If there's an incoming SMS, then check for "Check" ! GetSMS (); } for (int i=0; i<200; i++) { // clean up the serial reply buffer, to stop repeating actions if nothing received. Reply[i]=' '; } if (Count == 1000 && Status !=0) { // if the loop counter is 1000 and the Status counter is not 0, then resest the frequency and voltage stats, and decrement the Status counter. ResetStats (); Status --; } if (Count == 1000) { // If the loop counter is 1000, then reset the stats and get the signal strength. GetSignal (); ResetStats (); } } /* * Copyright (c) 2015 Andrew Doswell * * Permission is hereby granted, free of charge, to any person obtaining a copy * of this software and associated documentation files (the "Software"), to deal * in the Software without restriction, including without limitation the rights * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell * copies of the Software, and to permit persons to whom the Software is * furnished to do so, subject to the following conditions: * * The above copyright notice and this permission notice shall be included in * all copies or substantial portions of the Software. * * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE * AUTHOR(S) OR COPYRIGHT HOLDER(S) BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN * THE SOFTWARE. */

If you're using a SIM900A module outside of Asia, you may run into difficulties, as I did. There's some brief notes I made here : http://andydoz.blogspot.co.uk/2015/10/sim900-and-sim900a-module-signal.html and a link to a website which contains detailed instructions on sorting out the firmware to make the unit work.
Here's some pictures of my unit:

BTW. Sainsbury's is my service provider!

Electrical Safety is paramount. Here is the incoming mains earth (ground) securely tied to the chassis of my metal case. 

WEM Watkins Copicat, repairs, renovations and electrical safety upgrades!

Minding my own business, when the telephone goes, and it's a friend with a wobbly Watkins that no longer works.

Cabinet's OK. Shabby chic. Hinges need replacing...

Obviously heavily gigged in smokey pubs! Tape path is dirty, and the heads are worn, but they'll go for a while yet.








Most of the caps in the supply are in poor condition. These two were replced, the originals being high ESR and low capacity. There were also two caps decoupling the +/- 12 volts rails. One was short circuit.

One of the main problems with the copicat, is it's lack-luster electrical safety. It was made back-in-the-day when standards were different. There's a nice metal box, the chassis of which is used to ground the audio incoming and outgoing. Now here's the issue. It's not grounded to a safety electrical earth. There's only a 2 core mains lead fitted. Now in this day and age , this isn't good enough. Not unless special precautions are taken (double insulated). This certainly isn't double insulated, and, although a remote possibility, if a live wire were to come off the motor or mains transformer, and connect with the metal chassis, the ground of your guitar would now be live. It's going to hurt, or kill. Not good. It HAS happened.

So, what can we do about it? Easy... fit a three core lead, and ground the chassis. Brilliant. Guitarist now safe. Except for the thing now hums like billy-o, because we now have an earth loop. More here.

OK, so what can we do? Remember the isolation transformer we fitted to the Ekco radio? Just the job. This will prevent any earth currents flowing , and prevent the thing humming whilst maintaining electrical safety. 

Here, I've fitted the isolation transformer , and a nice new 3-core mains lead. The Earth is secured to the chassis, using a soldered ring terminal, and a nut, bolt and star washer to ensure good contact

The tape-tensioning mechanism is stripped, cleaned and lubricated.

All cleaned up and ready for the next 30 years of service!