Need to detect momentary switch and switch 12V, simply

[iso9001]

Hi,

I have a circut that has a momentary switch and a lamp.

I need to detect when the switch is pressed (either using a PIC or just a hardware circut) and then switch 12V to the lamp.

I know I could do it using a pic to watch for an falling edge on the switch line, and then use a push pull transistor setup to drive 12V to the lamp.

But I would like to keep the complexity and parts cound to a minimum.

Does anyone make a pushpull in a single package ?

Or is there a better idea to do this ?

 

[ljcox]

You could either use a latching relay or a flip flop.

The switch would be connected to the set input of the flip flop with a pull down resistor (assuming the set is active high). The CD4027 would do.

The Q output would drive a transistor to turn the lamp on.

You could also have another switch connected to the reset (or clear) input to reset the FF to turn the lamp off.

Len

 

[iso9001]

Yea, I was looking at flip flops just as I was reading your post.

I cant find one with a low enough pin count (<= eight), I dont mind the money, but space is kind of an issue.

 

[audioguru]

An 8-pin dual opamp can do it like this one:

 

[bonxer]

Yea, I was looking at flip flops just as I was reading your post. I cant find one with a low enough pin count (<= eight), I dont mind the money, but space is kind of an issue.

Earlier you mentioned it would be acceptable to use a PIC. How about a 12F509 or 12F629, those are both 8 pin PICs.

 

[Nigel Goodwin]

Yea, I was looking at flip flops just as I was reading your post. I cant find one with a low enough pin count (<= eight), I dont mind the money, but space is kind of an issue.

Earlier you mentioned it would be acceptable to use a PIC. How about a 12F509 or 12F629, those are both 8 pin PICs.

You can also get the 10F series in six pin SM - really small!.

 

[iso9001]

Oh no no, I already am using a pic. I just didn't know if I should 'waste' my interupt pin on a feature that is far less important then the rest of the pic is doing.

I just didn't know if there was a trick way to do this. Like some sort of momentary switch lamp driver ic thing.

The push pull just doesnt appeal to me because i need to add two new part numbers that look identical. I really would have thought someone would have a need for a specialized pushpull single package transistor

 

[Nigel Goodwin]

Oh no no, I already am using a pic. I just didn't know if I should 'waste' my interupt pin on a feature that is far less important then the rest of the pic is doing.

All you need is the switch and a pullup resistor (or not even the resistor, if you use the pullups on portb feature). There's no requirement to 'waste' an interrupt on the switch, simply poll it occasionally - this also helps with the debouncing required - PIC's are so fast (compared to button presses) polling is usally plenty fast enough. Even if you use interrupts, the interrupt routine usually just sets a flag, and the main program polls the flag - so why not just poll the switch?.

 

[iso9001]

Ah, I forgot to complicate things,

Its a 12V signal that goes to the switch. The one I need to poll.

Think its O.K. to use a large (>500k) resistor and pass the 12V signal to directly to the pic ? Or is that going to screw stuff up ?

 

[Nigel Goodwin]

Ah, I forgot to complicate things,

Its a 12V signal that goes to the switch. The one I need to poll.

Think its O.K. to use a large (>500k) resistor and pass the 12V signal to directly to the pic ? Or is that going to screw stuff up ?

You can feed a 12V signal into a PIC pin with a simple series resistor, the protection diodes in the PIC clip it to 5V - the resistor stops the diodes passing too much current, the spec for the diodes is in the datasheet. You don't need as high a resistor as 500K - a 68K would only pass 100uA through the diode.