one pulse switch...

[bubulindo]

I need to build a one pulse switch, that is, i need to turn on and off relay with a pulse button.

Basically it's this that i need.

_ _
Button ___| |_________| |__
_____________
relay ___| |___


reply to ee01013@fe.up.pt

 

[nbucska]

Use an LS74 D flip-flop with not_Q connected to D with a 1Kohm and connect a 100microF from D to GND ( - side to GND)

If you need more current than about 10mA, add a transistor
(e.g. 2N2222 : drive base with 2Kohm from Q and add a diode
parallel to coil in the collector ( Anode to coll, cath. to
+V )


<nbucska@pcperipherals.com>

 

[melone]

Or just use a push-to-make / push-to-break switch...

 

[itsmeRICK]

I think bubulindo needs a monostable. A monostable gives one pulse of a preset durration in response to a triggering pulse.

You can use an 74HCT123 or even the old NE555 could to the trick. You should use something like a 2N2222 as a buffer as inbucska sugested.

One place you can find some examples (probably one of hundreds) is:

http://ourworld.compuserve.com/homepages/Bill_Bowden/page9.htm#555mono.gif

If you scroll down to &quot;555 timer Mono stable (one shot) circuit&quot; you will see the circuit I am thinking of.

Good luck.

Dick

 

[bubulindo]

I've tried this circuit but i've got a problem.
Every time i turn the circuit power off and on again, the switch may be on or off.
How can i ensure that every time i turn off and on the power in this circuit, the first state will be off ???

Best regards, Carlos Cardoso

 

[Skogsgurra]

bubu,

You need a POR (Power On Reset) circuit.

A simple one is a capacitor and a resistor. Connect the capacitor to the V+ and the resistor to the GND and the common point to the R input of your bistable.

When power is applied you will get a short positive pulse on the R input, which will keep your bistable in the OFF state.

If your reset input needs a zero to activate (that is a /R input) then connect the resistor to the V+ and the capacitor to GND.

If you are using CMOS then any reasonable resistor (1 kohm - 1 Mohm) can be used, the capacitor can be choosen even more freely. A good working combination is 10 kohms and 100 nanofarads. It will give you a 1 millisecond time constant and the reset pulse will be in the same order of magnitude.

If your supply voltage has a very long rise-time, you will need a longer time constant. Select a 1 microfarad capacitor and a 100 kohm resistor.

For TTL, you will need a much lower resistor (100 - 330 ohms) and a higher capacitor value. Go for electrolytic capacitors if you need more than a few microfarads.