Circuit to trigger on an voltage spike

[TimB]

Hi all

This is a random one. I have an RF doorbell that is not power full enough to get down to the new workshop repeatedly. I have another one that I have dismantled and figured out that if I ground the button on the push button it fires off.

What I want to do is make a repeater so when the receiver of the main bell goes of and plays its melody it triggers the second units button eg pulls it low.

Using a pic it would be simple. Use a IOC pin and pull a line low when it triggers.  I can add in delays so it will only do so after a set period eg 3 seconds to let the melody finish.

However I do not have a little pic lying around and with out building a board it would be a pain to program. So looking for a simple transistor set up. I'm sure a transistor can be made to do the job but not sure on how to make the delay.

Spec I have to work with
0v and 4.7v on receiver
Activation Sound is DC  SItting at 0v then spiking at around 2-3v going to the speaker.

Cheers any ideas

Tim

[top204]

Do a search using "transistor monostable delay -555" Tim.

It will show quite a few ways of creating a delay with a transistor or two, but the principle is keeping the transistor biased enough to be on using an electrolytic capacitor, and the larger the capacitance, the longer the delay is because it keeps the base voltage higher for longer. Or the reverse if the capacitor is attached to VDD.

The site below has a few circuits, and they can be adapted for less or more transistors. It just depends on what is actually being switched:

Delay ON Timer circuit using transistor

[TimB]

Thanks Les

Thinking about it the effort to mount a 555 and the comps etc I might as well use a small 8pin pic. If I use a dil version I can use a breadboard to program it then make it a dead cockroach style mounting and glue the back to the existing PCB

Thanks a lot though.

Tim

[JonW]

How far is it?  1300ft doorbell

If you know the frequency of the doorbell the RX & TX are likely to have a really poor efficiency printed or coiled antenna.  You could add a length of wire on each end to make it better.  I would opt for £25, but if you want to have fun try the repeater

[TimB]

Hi Jon

That's the doorbell I have. It does reach some times but if the weather changes it fails

Tim

[JonW]

Improving the antenna will help alot.