555 Won't "Time Out"

I'm trying to drive a relay using the 555 chip (actually RS NE5567). The power supply is regulated 6 volts, 1 amp and the relay runs on 5 volts. Relay is polarity sensitive, has 175 ohms across coil, and draws 32 ma (both as measured).

I'm using a 1 meg resistor with a 10 mfd cap and it times out beautifully at 11 seconds using an LED/150 ohm resistor on pin 3/gnd. However, when using the relay, it does not time out and stays latched until reset or power is cut.

I've tried a diode across the resistor and/or added more resistance to no avail.

Any ideas?

Relay is a OMRON 1574Y1, G6H-2, 5 vdc

Posted by Anonymous on 17/07/2003, at 19:38 GMT

Suggestions to cure 555 monostable timeout failure

The problem is probably feedback through the power supply as the relay as switched off. There will be a small but sudden change in the supply voltage which may be confusing the 555 comparators.

These are some things to try:

1. Check that you have a signal diode (reversed biased) across the relay coil, eg 1N4148.

2. Add a 0.1uF capacitor across the power supply near the 555. This is in addition to the smoothing capacitor(s) already present on the supply.

3. Add a 0.1uF capacitor in parallel across the relay coil.

4. Try changing the timing components to a 100k ohm resistor and a 100uF capacitor. Lower value timing resistors tend to make the 555 less sensitive to noise and they make any leakage in the electrolytic capacitor less significant.

Posted by John Hewes on 18/07/2003, at 15:21 GMT

Thanks

Thanks, after further experimentation I found that a 10 mfd NP cap across the relay coil solved the problem and the system now works fine as intended with the 11 second delay.

Thanks again.

Posted by Anonymous on 18/07/2003, at 17:20 GMT