View Full Version : Low battery wireless alarm

14-08-2010, 13:20
Electric engine speed controllers for model aircraft protect the onboard LiPo batteries by shutting down before the voltage drops to a damagingly low level. This can result in a long walk to recover the aircraft if (like me) you find it difficult to judge how much battery time you have remaining. To overcome this problem I developed the following circuitry to monitor the aircraft battery in flight and send an alarm signal, via a low-power 433 MHz wireless link, when the voltage approaches the speed controller shutdown level. A small belt-mounted receiver then flashes an LED and emits a buzz to warn the RC pilot that its time to think about landing. This is about as simple as telemetry gets and the system can be implemented using a low cost transmitter-receive pair, that are available in the UK from Maplin Electronics (part number VY48C) for £10 +VAT, and a couple of 8-pin PICAXE chips.

The airborne circuitry is shown at the top of the attached schematic. The 08M generates 950Hz tone burst of one seconds duration to drive the ‘data’ input of the low-power transmitter. The repartition rate of these bursts is normally once every ten seconds but increases when the battery voltage, monitored by ADC input AN-4 via potential divider R1/R2, drops below a preset alarm level. Note that the PICAXE supply voltage will remain regulated until the battery drops to about 5.5 volts. The 08M code for the transmitter is shown below.

pause 1000 'Keep tone off for 1 second
readadc 4, b0 'Monitor battery voltage
if b0<141 then goto alarm 'Compare with minumum setting of 8.8 volts
pwmout pwmdiv16,2,65,132 'Generate 950 tone
pause 3000 'Keep tone on for 3 seconds
pwmout 2,0,0 'Turn off tone
pause 10000 'Keep tone off for 10 seconds
goto monitor 'Loop

pwmout pwmdiv16,2,65,132 'Generate 950 tone
pause 3000 'Keep tone on for 3 seconds
pwmout 2,0,0 'Turn off tone
goto monitor 'Loop

The pilot-mounted receiver circuit is shown at the bottom of the attached schematic. The 433MHz receiver module outputs TTL level pulses that drive a piezo sounder and the count input of a 08M. Components R5/C4 form a low-pass filter at the Schmitt input (I/O2), which reduces the effects of high frequency noise. The PICAXE turns on the LED if the received signal falls within a present frequency band of 900-1000Hz. A low burst repartition rate indicates that the telemetry transmitter is still within range and the higher rate warns of a low battery voltage. With the model is airborne; the range is several hundred feet using ¼ wave whip antennas for the transmitter and receiver. Range could be improved by replacing the 08M tone detector with a LM567 phase lock loop, due to its increased sensitivity and filtering characteristics.
The 08M code for the receiver is shown below.

count 2, 1000, w0 'Count input pulses for 1 second
if w0 <900 then goto LED 'Lower limit
if w0 >1000 then goto LED 'Upper limit
low 4 'Turn LED on
goto start 'If frequency is within limits, loop with LED on

high 4 'Turn LED off
goto Start 'Loop with LED off

Note that in circuit programming components are not provided as this function was performed on a separate board.

14-08-2010, 13:26
Nice and neat MFB.

I've never heard of the term "repartition"?
What is that?

14-08-2010, 16:21
Dippy. Please forgive my spelling. I am a Bristolian and English is therefore not my first language

16-08-2010, 11:44
Some information that I should have included, when mentioning the LM567 tone decoder option, is that this chip should be driven from the pin 13 of the 433MHz receiver module. Pin 13 outputs the raw analog signal before the bit-slicer stage that is used to produce TTL pulses. This configuration allows the LM567 to pull inaudibly low level tones from very noisy radio links.

08-10-2010, 03:39
thank you for your post !!!
I am using a laptop and low battery wireless alarm is really necessary for it.

13-10-2010, 20:59
Great job.

Just a question though, why not drive the TX module from a PICAXE output pin? This way you will have power control also, by only switching the module on when you need to.

25-10-2010, 17:07
It might well be useful to power down the transmitter for some applications. However, the current taken by the alarm circuitry is insignificant compared with the aircraft motor.

I have found one potential problem though. The receiver must be operated several metres away from the 35MHz RC transmitter, in order not to overload the input stage of the 433MHz alarm recevier.