Timer circuit

[myp71]

What whould be the best way to build a cheap simple timer circuit using in a low M.A. automotive app that would countdown without a pushbutton switch.So just apply power to the timer circuit and it counts down with the power applied then resets when there is no power on the input side of the timer.Time could be around 3-4 seconds<p>Thank in advance
Myp71
Ryan

[Chris Smith]

The 555 is the best for simple solutions.

[myp71]

Thanks for your help but if you could provide me with a digram on how to hook it up, you would be even a greater help<p>
Thanks in advance
Myp71

[greg123]

Check out this link.<p>I know i have PWM written there but it is the same thing.<p>Timer Circuit<p>greg

[myp71]

One more quesiton do I still use the pot and if i do how do I connect it up? Thanks a lot for your help Greg and others that have/will help<p>
Thanks again
Myp71
Ryan <p>[ May 15, 2003: Message edited by: my p71 ]</p>

[bridgen]

Ryan, I read your requirement to be a circuit which produces an output which stays high for 3 to 4 seconds when power is applied, then does nothing until power is applied the next time. <p>If that is correct, Greg's circuit, which produces a continuous series of pulses for as long as power is applied, doesn't help you. <p>If I'm right you need a monostable which is triggered when power is applied. Let me know. <p>Regards.

[myp71]

I see where my problem is. I would like to hook up a chime circuit (I already have the chime)that would be activated when I apply my parking brake so that I know I put the brake on and won't forget.I don't want a continous chime, rather something that will shut off power to my chime after a couple of seconds. I would like to apply the trigger switch right into the parking brake circuit. I need a circuit that would trigger my chime once only per applying my parking brake cycle. In other words, I put on my parking brake, my chime is activated for 2 or 3 seconds, then it stops. The next time I apply the parking brake, the chime cycle starts again. <p>Thanks for your help
Myp71
Ryan<p>[ May 16, 2003: Message edited by: my p71 ]</p>

[bridgen]

Hi Ryan,<p>that's a bit more to go on, thanks.

You mention the parking brake "circuit". Does this mean that it already has a switch - which maybe puts on a light on the dash? <p>If so, does it connect the light to chassis to complete the circuit (the bulb already having 12V at the other side), or does it connect 12V to the bulb (the other side already being connected to chassis.)?

[Chris Smith]

You have no Email Address? <p>Ive got many,..... other wise, from Forrest Mimms, the simplest and best.

[myp71]

O.k , there is already a switch for the Light on the dash which is negitive switching so I will just tie into that right?and yes it does connect the light to chassis to complete the circuit the bulb already having 12V+ at the other side of the light on the dash<p>Thanks for all your help guys it's really helpful<p>Myp71
Ryan<p>[ May 16, 2003: Message edited by: my p71 ]</p>

[bridgen]

Hi Ryan,<p>Yes that's what I had expected. <p>I was going to draw a diagram and post it on my web site for you but for reasons I don't understand I can't connect to the host from my web editor - infuriating.<p>So, let's do it the hard way. <p>Uing a 555, pin 1 to ground, pins 4 and 8 to 12V. <p>Connect pins 6 and 7 together. From these, connect 470k to 12V. Also from them connect 10uF to ground. <p>Connect your chime between pin 3 (the output) and ground. <p>Connect 100k from pin 2 to 12V.<p>Connect a 0.1uF capacitor from pin 2 to the junction of the dash-mounted light and the switch which grounds it. <p>Pin 2 will then normally be held at 12V through the 10k. When you apply the parking brake, pin 2 gets briefly grounded through the capacitor and triggers the monostable which then produces an output for around 4 seconds. <p>You can play around with the values of 10uF and 470k on pins 6 and 7 to adjust the time. <p>If you're using the 7555 (cmos version) you don't need to de-couple pin 5 to ground, but you are limited to the amount of current which pin 3 can source. (Check the data sheet.) <p>On the other hand, if you need more current from pin 3 (up to 200mA), use the bipolar 555 and de-couple pin 5 to ground with 0.1uF. If the chime is inductive you also ought to put a diode, e.g. 1N4001, in series with it from pin 3.<p>Regards,<p>David

[myp71]

David,

Thanks so very much for your help.Really nice of you to spend all your time on this for me!This circuit is really going to help me. <p>Thanks
Myp71
Ryan<p>[ May 17, 2003: Message edited by: my p71 ]</p>

[gadgeteer]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Connect a 0.1uF capacitor from pin 2 to the junction of the dash-mounted light and the switch which grounds it. <hr></blockquote>Exactly how I woulda wired it. 555 is NOT "edge triggered", so if you don't convert the switch signal to a pulse, beeper sounds forever.<p>I found once when I tied someone's brakelight switch to their horn relay, that an additional filter cap of ~.01 between pins 2 and 8 was needed to prevent false triggering. Man EVERYTHING he did honked the horn --- brakes, dome light, TURN SIGNALS. He was NOT happy with me...<p>If you don't need much drive current, a 4538 also works well @ 12v, and it IS edge triggered so you don't need the input resistor/capacitor...<p>[ May 18, 2003: Message edited by: gadgeteer ]</p>