Timer circuit problems.. help!

[letsrelaythat]

Hi All,

I have built the following circuit twice, and can't get it to work properly. I built it once on a smaller PCB, everything was very close together and didn't work, so I bought all the components again and built it on a much larger PCB, and spaced everything better.

The idea, is that I pass constant 12v+ to the top left wire, and when I pass 12v+ to the wire coming from the left in the middle, it should let the wire from the top right goto ground. This will most likely be a relay, used to keep a car running for a preset time, i.e a turbo timer.

The switch after the capacitor determines if the circuit is instantly broken or has a timed delay.

What happens is that the LED (which im using instead of the relay) comes on as soon as I give it 12v+, not when it has 12v+ and also the ON wire has stopped giving signal.

What should I look for on my PCB, any ideas where I may have messed up?

Thanks all in advance, :?

Tim.

 

[-=GST=- Nemisis (cs/cz)]

its a monostable (time delay) yes is it is then its toataly wrong, everything is messed up with ur connections by the 555, u have most probably blown it on ur pcbs :? here is what it should be, all you need to do is change the capacitance for the timing, also i have added that relay just for example, if its a astable (on-off-on-off-on-off) then i think its wrong but i didnt look for long enought, also you dont need all those diodes. it would hard work changing ur pcb into a monostable, ur best to un-solder the compoinents and then make a new one.

 

[letsrelaythat]

Well I was always under the impression that you should use a transistor to drive a relay, and that there should be a diode across the coil..

Apart from the diode and resistor down the bottom right of the circuit, and the transistor on the left, they are basically the same circuit..

Should the relay be done by transistor?? I have a bunch of 2n2222's that im using

Cheers all,

Tim.

 

[-=GST=- Nemisis (cs/cz)]

the reason it wasnt working is coz of all the wrong connectons on the pins of the 555. apart from that it has to be ur misconception of where each pin is on the actual chip, or that you made it wrong both times. the only other diode variation i have seen is one where from pin three a diode goes to ground. this does work i have tried it. if there is anything else you want to know just ask one note though, i dont ever use relays, too expensive, so i dont knwo what the coil is. if a diode could be added there with no damage to the rest of the curcuit then do it, just as a back up.

 

[letsrelaythat]

Well I've checked every wire over again, and removed some diodes.. I've got it to work sort of, must have been something with the diodes affecting the circuit..

But now I have the problem that if, after I remove the ON wire, and put it back on, the circuit won't reset.. I remember seeing the 2 diagrams to fix this so I'll try to find it, but I have a suspicion that it uses diodes...

Also will the timer take 15v?? then I can remove these voltage regulators.. well at least 1 of them :roll: :roll:

Thanks all,

Tim.

 

[gerty]

**broken link removed**
Maybe this'll help answer some of your questions. It has several 555 circuits, and calculators to help you figure the r/c values

 

[letsrelaythat]

Well I used this site to build my very original circuit, I used the Various LM555 Time Delay Circuits Circuit #2 - Negative Recovery.. this didn't do what I wanted it to do, so on another site I read a bit more into it, then ended up connecting pin4 (reset) to the +14v as well..

This works and I am slowly redesigning the circuit for the 3rd time, using hardly any relays and keeping the components to a bare minimum..

The circuit at the moment turns the relay on when I take away the 14v+ from pin 2.. It then won't reset, i.e it is non-retriggerable.

Anyways I'll give it all another go, thanks for all the help.

Cheers,

Tim.

 

[williB]

did you try this circuit?

 

[letsrelaythat]

No because of the note at the bottom. I need it to hold the relay open until a certain time after I have removed the 12v from the ON wire, but continually reset the timer until I take the 12v+ off.

I have in the meantime found a much simpler idea, by using a capacitor, a-like-a-so. One question, how do I vary the time it takes for the circuit to close the relay, by using a variable resistor..

Thanks in advance,

Tim.