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## Using an MCU to send an SMS message with the keypad

I’ve been looking at GSM modems and wondering how I could integrate them into my projects, they are around \$50 or so (Sparkfun) to play around with.  I’m thinking of using them to send an SMS when an alert is detected for a sensor, this could be a smoke detector, intruder alarm, temperature rise, etc. All you would need is a pre-paid SIM card and some credit to last a few months.

With everything, I want to use something that I already have and in this case I have some old mobile phones. It’s crazy to think that you can buy new cheap mobile phone now for around \$26.

One of the old phones I have is a Nokia 3120. I’ve read there is a MBUS/FBUS protocol which you can use to send or receive SMS’s however my attempts to send any data to the phone fell on deaf ears, I could probably figure out what I should be sending if I had a data cable to sniff the traffic. So I was left with the only thing I could do, wire up the keys and make our MCU press them for us.

## Verifying the thermistor function for temperature reading

I recently received a question in regards to the thermistor formula found on one of the SATL build posts asking how I came about the variables used in the thermistor formula. The thermistor formula and variables I used can be found on the Arduino Playground however it made me think if the formula being used was accurate for the Vishay 10K thermistor I was using.

Below we have the Thermistor function in question in which it converts the ADC value to a resistance value in ohms, applies a formula and convert the result to celsius.

```double Thermistor(int RawADC) {
double Temp;
Temp = log(((10240000/RawADC) - 10000)); // Minus by 10K as that's the resistor in series with the thermistor
Temp = 1 / (0.001129148 + (0.000234125 * Temp) + (0.0000000876741 * Temp * Temp * Temp));
Temp = Temp - 273.15; // Convert Kelvin to Celcius
return Temp;
}```
##### Steinhart–Hart equation

The formula used is called the Steinhart–Hart equation which models resistance to temperature, there is the “full version” and the “commonly used” one. A, B, C and D in the functions below are called the coefficients.
Full version: 1/T= A + B*ln(R/Rt) + C*ln(R/Rt)2 + D*ln(R/Rt)3
Commonly used: 1/T= A + B*ln(R/Rt) + D*ln(R/Rt)3

## Inside the Cyberguard Snapgear SME530 VPN Firewall Appliance

Today we’ll be looking at the Cyberguard Snapgear SME530 VPN Firewall Appliance which is used between modems/routers and the network, kind of like a UTM. Cyberguard were bought out by McAfee and then discontinued.

Two screws later and we’re in. If you look near the bottom right, you’ll see a battery probably for the RTC. Interestingly enough the Netgear UTM which I took apart also had one too but routers/modems don’t have them, I guess these kind of appliances where logs are everything you don’t want to re-set the date/time when they power down or reset .

## Measure AA batteries for low voltage: Low Voltage Battery Monitor

I have some rechargeable batteries which are on their way out as they only seem to last 1-3 weeks but I never quite know when they are flat when they aren’t being used. So Instead of buying new rechargeable batteries (which is the easy solution), I thought why not make a simple project out of it.

(Testing with only 1 AA battery)

Introducing the Low Voltage Battery Monitor (LVBM) – a 4x AA battery monitor which will blink an LED every 4 seconds if a battery is found flat and checks good batteries every 15 minutes.

##### AA Battery drop off voltage / ADC

Firstly we need to find the voltage when an AA battery drops off, we can find this on an AA battery datasheet: http://data.energizer.com/PDFs/nh15-2300.pdf.