timed open circuit on normally closed

Hi guys, first post so be gentle :P

I think this is the right area for this, but anyway:

I've got a reverse beeper warning in my IS200, I'm wondering if there is a way I can time it off. I.e. it currently remains permanently powered until it comes out of reverse again, and I'd like it to stop beeping after a few seconds, regardless of power through the circuit still. As far as I am aware it is a simple power, buzzer, switch, ground circuit so it shouldn't be that complex to install anything inline. Am I right in thinking that clever use of a relay and capacitors might be able to do this? Or would a locking relay or bi-metallic strip be a better option?

Any help would be greatly appreciated. Thanks

Use a timer relay simplest way, e.g. DEI 528t or a Velleman kit.
Simpler and more reliable.

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Test before boxing up.

MMM Thats definitely worth looking into. After a bit of googling I think the term I should be using is Auto-off :P Why that didn't occur to me earlier I don't know, but hey, live and learn. I did some reading about the 555 timer, can that be used effectively in this situation?

Yes, both the above kits use a 556, the 528t is ready made so far simpler.

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Test before boxing up.

Just had a play with a circuit builder, would this not work with the correctly sized resistor and capacitor? As the reverse switch is closed, the capacitor will begin to charge, closing the relay and allowing current to flow to the speaker, and eventually closing the relay when fully charged after a few seconds, then as the reverse switch is opened again, the capacitor will discharge, closing the relay, but the speaker wont sound as the reverse switch is open now.

Is this correct? I don't particularly want to mess with timer circuits for something like this :P

Very much so, I generally use pre made because they're more reliable.

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Test before boxing up.

Excellent, just had another play, I think I'm going to be working in a confined space, so a relay might not work as well. Will the circuit on the right do the same thing?

Not sure but have you looked at RS for mini power relays PCB mount?
1-5 amp are very small and I'll guarantee the buzzer draws about 1/2 amp or less.

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Test before boxing up.

I haven't no, I'll have a look and see what it comes up with, thanks

Q1 needs to be NPN. It won't be a sharp turnoff, but IMO it's better than relay chatter.

Thanks oldspark, I never fully understood the practical differences between npn and pnp. I know the makeup and how they work, but never got what difference they made to a circuits behaviour

Actually (LOL...) I'm tempted to bet you know more than me!

I merely knew - or applied the general rule - that the arrow in a symbol denotes the direction of travel of conventional current - ie, from +ve to -ve thru the load or device.
Your arrow direction did not allow current to flow thru the load (speaker), hence it was wrong.
Since I also recall the most common transistor (NPN) as having the arrow outward, therefore yours was PNP.

Ok, I also knew a bit extra - like Vbe must be 0.7 or above for conduction and Vbe is +0.7 for NPN (versus -0.7 for PNP) - and applied that... tho that too was essentially rigged "graphical memory"...

But notice my lack of confirmation that it will actually work? (No doubt spotted by experts and clear thinkers...)
But according to my SIMPLE logical and graphical analysis it will.

However, it is likely to be a soft or gradual turn off as any RC decay or rise circuit is - unless something causes it to be sharp. (IE - hysteresis thru clever circuitry or device characteristics; follow-on Schmidt triggers, etc.)

And hence - to avoid the need for clever design, I agree with the use of a square wave (on-off) timer circuit - like DEI or 555 timers etc.

FYI - my added misc comments include:
- I disliked timers like DEI, but hey - they are relatively cheap; well designed (versatile); and require no assembly etc;
- I often suggest the PICAXE 08 (08M2) in lieu of 555s because they are almost as cheap; use FAR less current; and are programmable (they have ADCs, a DAC, i2c comms interface; PWM outputs for dimming, etc).
- But I certainly approve of your 'buffer' transistorised circuit with small C rather than a brute force big C direct on a relay coil.
(IE - big kudos to you on your circuit!!! )


Anyhow, the above is FYI - and largely to offset your OP's "...first post so be gentle... ". Hey man, that works both ways! (At least in MY case.)