Hello forum,

I have been designing a circuit for a RC failsafe (puts throttle servo in brake position in a nitropowered RC car if there is radio-problems) and I want to hear your opinion on my component choice, resistor value choice ect.

Quick circuit explanation:

The MAX1726 is a 5V low dropout 20mA regulator.
R7 enables reverse battery protection in the MAX1726.
IC3 is a ZR431 2.5V shunt regulator (used as Vref).
R6 will current limit the input signal if this should be higher than the PIC's Vdd.
R8 and R9 is a voltage devider, to measure the battey voltage using the ZR431 as Vref for the internal AD converter ind the PIC.

The circuit will be powered by 4-5 NiMH cells.

Thanks for any suggestions 8)

Regards,
Futterama

Attached Images

circuit_939.gif (11.5 KB, 210 views)

 

I didnt review your circuit (sorry no time) but ask yourself this question, if any of these components fails (including misassembled), is the design safe? If your answer is no in any instance, it is not fail safe.

 

Unless I have multiple circuits working in parallel, it's not 100% safe...

The expression "failsafe" is used not to describe the circuit itself, but to describe the function of the circuit, e.g. the circuit will keep your RC car safe in the event of radio problems (e.g. loss of signal), and ofcourse, the failsafe unit will have to be testet from time to time, in safe conditions (when the nitro engine is not running) to increase the safety.

I was looking for comments on my design, if I have missed something, or perhaps overlooked something, or even miscalculated a resistor value.

Thanks.

Regards,
Futterama