Help needed to make 555 timer circuit throw relay.

[atsijs]

Hi,

I'm trying to make an automatic control circuit for a model railway shuttle. I've read the principles elsewhere and it all sounds good in theory. There are basically three parts to it - all of which I have working independently, and two together, but I can't get the last one to click (literally).

So I have set up my track circuit (using isolators and diodes to stop the train running off the end) and my switching circuit (a DPDT relay which reverses the polarity on the tracks - this works with a 9v battery applied directly to the relay).

The idea was to use a 555 timer to throw the relay periodically. I bought some cheap generic 555 kits, the circuit of which is here:

http://www.rapidonline.com/pdf/70-0225.pdf

I've put this together in the astable configuration and it makes the LED flash at the intervals I'm after (after some switching of capacitors and resistors).

I naively assumed I would simply be able to connect my relay to the 'output' on the circuit. Apparently the pin 3 current is not sufficient. I have tried using a transistor off the pin to amplify the current to no avail. I've read something about the need to put in a diode but that makes no sense to me.

Please can anyone tell me if this is possible, or whether I've just wasted a few evenings trying to make it work (albeit enjoyable and educational evenings). Or am I just doing something wrong that should be obvious to me?

Thanks in advance,

Kev

[Hero999]

A 555 timer can drive a relay as long as the coil current is below 200mA. A diode needs to be connected in reverse parallel with the relay coil to protect the 555 timer from the high voltage pulse generated when the relay is switched off.