# How can I have a rising edge close a relay for a fixed time?

I have a problem that I don't know what the solution is called, but I would bet that there are many solution.

Problem: I have to short-circuit two pins on a mainboard (power switch) when a rising edge on a control wire (voltage is turned on and stays on) is detected, but these two circuits have to be separated without any direct connection, so I can't just plug the computer on the voltage but have to use some sort of relay, which closes two contacts (but only for a short time) when powered.

The wave form would look something like this:

What is the name for this type of circuit?

• What voltages are we talking about? and what kinds of currents? And what is the purpose of this - there may well be a power management IC that will do what you are trying to achieve. Jul 12, 2011 at 12:43
• I tried to help make the English of your question more clear. I hope I didn't change the meaning of it at all. Jul 12, 2011 at 12:52
• @Matt Jenkins from what I gathered he is trying to fake the push of the power button on a computer, so practically no current. Just needs a quick press of the button to turn it on. Jul 12, 2011 at 12:54
• If only small currents are involved you might want to think about possibly an opto-isolator instead of a relay. This will run silently (no click), consume less power, not create any flyback voltages, and possibly have a longer life span than a relay. Jul 12, 2011 at 12:58
• @Matt - Well, I offered you make your comment an answer, and I gave you credit. (yes, I saw the smiley.) Jul 12, 2011 at 13:42

The solution is called a monostable multivibrator, or MMV. When it detects a rising (or falling) edge it gives a pulse at the output with a chosen duration. You use that pulse to trigger the relay. This is a typical job for an LM555 timer.

If the signal to be switched is DC you can use an optocoupler, like Matt suggested. A CNY17 may be a good choice. If the signal is AC you could use an SSR like the LH1500, or a reed relay. I wouldn't use a power relay because they're not made to switch low current signals.

• If you fancy building your own monostable from discrete components it's actually very simple. Wikipedia has a small example circuit here: en.wikipedia.org/wiki/… Jul 12, 2011 at 13:38

You want a edge to glitch converter, also known at a "one shot", and at black tie and tuxedo affairs as a "monostable multivibrator".

A very simple form of this is a R-C high pass filter feeding a logic gate. There are also deliberate one-shot chips, like the 74x121, but that's very klunky by today's standards.

Nowadays this sort of timing is done in a processor. There's usually a processor around somewhere, so adding a fixed length pulse output is usually simple. Since you're driving a relay, you're going to need 10s of ms timing, not microseconds, so the timing can easily be done in a 1 kHz periodic interrupt routine you probably already have.

Processors are real cheap now, so dedicating a tiny and cheap processor to this wouldn't be out of line. A PIC 10F200 can do this job with a few % accuracy. All you need is the PIC, which comes in a SOT-23 package, and the bypass cap.

Anyway, the point is there are lots of ways of doing this. Without more particulars of your situation, like how long the time delay, how accurate, what power is available, voltage levels, etc, there is nothing specific to recommend.

• Wow, I didn't know I was "black tie"! :-) Does this mean the 555 is actually dead and buried? It's still only half the price of the cheapest uC. (congrats on your passing the 5k in such a short time) Jul 12, 2011 at 13:48
• Getting good. Digikey 10,000 quantity. NE555 10 centes. 10F200 34 cents. Expect to halve the '555 price in Asia and even Digikey will probably offer you processors nearer 20 cents in private. CD40106 = 12.2c/10k Digikey - and you could make 6 x simple monostables if you really had to :-). Add a driver transistor for each probably (about 1 cent each). Jul 12, 2011 at 14:01
• @Russell: This shows that there are many solutions that are all effectively 0 cost unless this is a high volume product. For anything less, you can pick what you want for other unrelated reasons. Jul 12, 2011 at 14:15

A Steven says, the solution is usually termed a "monostable multivibrator" or just monostable in many cases. There are many means of implementation but a quick cheap easy (chose any 3) way that will probably meet you need is:

Assumes: Applied 0/1 on Voltage shown in diagram is suited to operating a relay AND that it is also accessible as a permanent DC level.

[1] Relay only solution - no electronics.

Uses:

• Two relays

• Electrolytic capacitor large enough that when charged it is able to operate a relay for long enough to provide required output pulse,

• Power source suited to operating relays = Vcharge.

Relay A has a "changeover contact". Relay B has a single "normally open" contact.

Relay A changeover contact is connected so as to connect the capacitor to Vcharge when un-operated and to swap capacitor onto coil of relay B when operated.

1. System idle. Capacitor is charged via Relay A contact

2. Input signal occurs. Relay A operates. Contact on Relay A connects capacitor to coil of Relay B.

3. Relay B operates, powered by capacitor. Relay B contacts activate output circuit. Relay B remains operated until Vcapacitor falls to Relay B dropout voltage. Relay B then releases

4. A operated, B released. Stable power on state.

5. Power to A is removed. A releases. capacitor recharges for next activation.

[2] One relay plus low cost transistor

Instead of driving Relay B directly, contact A connects a charged capacitor to the base of a transistor via a suitable resistor. Transistor (MOSFET of biploar) is used to operated relay B. Operation is as before except the small magnitude of the base or gate drive is well below Irelay so longer delays or smaller capacitors may be achieved.

[3] Superb - one relay, low cost, "Friend for life".

A package of 6 x CMOS Schmitt inverters will allow this to be done with 1 inverter (1/6th of package)(Maybe 2 if you want extra features.

Notionally the same as solution [2] but far more flexible and powerful.

This needs only ONE relay. Relay B is used (if desired) and relay A is replaced with a gate based monostable. Very cheap and easy. You'll probably need a single transistor to drive the relay (or an extremely sensitive relay). Circuit components apart for relay is < \$US1.

You can buy a formal monostable IC such as the 74C221 [1] BUT once you learn to use CMOS Schmitt inverters you will have a friend for life. A pkg of 6 such inverters is possibly the most useful analog timing IC available. Useful for clocks, delays, monostables (retriggerable and one shot), PWM modulators, tone generators, simple switching regulators (I have one in commercial production) ... .

All in a 50 cent (in 1's [2]) IC . That datasheet is for a CD40106 but there are various alternatives, some about the same in charactistics, some somewhat different. (74xx14, CD40106, xx4584, 74xx19, MC14584, ...)

I can provide a complete (and simple) circuit with values etc IF there is interest. No point in expanding the answer if not of value to people.