FAIL-SAFE

[Zlatan_Zivkovic]

This is the shematic of the FAIL-SAFE system for RC-models.
It's a simple servo controller wich activates when the signal from the transmitter is lost. It automaticly turns it self off when it recives the signal again.

How does it work:
receiver  ->  FAIL-SAFE  ->  servo motor

1.WHEN THERE IS NO SIGNAL:
The signal pin (lowerest pin from the J1->connector wich is connected to the receiver) is low (0 Volts) so the 555 timer can control charging (to 2/3 of +V - voltage source) and discharging (to 1/3 of +V) of the C1. 555 timer then sends pulses to the servo (wich is conected to the J2) (at fregency around 40Hz) wich are modulated by the R1 (100 kOhm trimer or potentiometer) (puls widht: 0.7-2.5 ms, and distance form the pulses is constant: around 24ms). Conect everything together as it should be, turn on the receiver and turn off the transmitter, then you should be able to set position of your servo by changing the R1.

2.WHEN THERE IS SIGNAL:
Pulses from the receiver (wich are also from 0.7 to 2.5 ms long, only the distance betwen them is a little bit shorter) keep the C1 charged (to +V) so the 555 timer doesn't have a chance to discharge C1 to 1/3 of +V. Those pulses are also sent to servo, so the servo is adjusted by the transmitter.

+V  - positive voltage source from the receiver( 4-12 V, usualy around 5 V)
DIODES  - can be 1N4001.......1N4007, or even 1N4148
J1  - connector to the receiver
J2  - connector to the servo motor
R5 - doesn't have to be in circuit for some servos, but for most servos it does

It's very small,cheep and efective device, response time is lower then 0.1s
I always needed one for my RC boat, so had an idea wich I tested in program and made my self one and it works great.