Weird timer circuit needed...

[Albany Tom]

This one's been driving me nuts. It's for a ham radio project I'm working on for a local club.

I have 4 pushbutton inputs, which need to be debounced, converted into a 2 bit binary address, and a pulse generated to trigger another circuit. All of this would be mostly simple, except that the address lines need to be stable at least 300ns before the leading edge of the pulse. Anything up to a few dozen microseconds would be fine, actually, but the second circuit needs that 300ns setup time. I'd like to do this with standard, off the shelf stuff.

300ns is more than normal IC delay circuits I've seen, but sounds too short for a 555. I'm thinking a "two shot" would work - anybody have any simple way to build one of those? Am I (probably) missing some really great simple way to do this?

In short - Help!

 

[UK Owl]

just a thought...

why not use a Logic gate based oscillator (relying on the 20ns or so delay in a logic gate switching), this should give pulses anything up to a 80-100 ns rate, then a counter circuit.

this should only take one cmos 4000 series (or 7400/5400 TTL) logic chip and counter.

p.s. excuse any errors in the times I have mentioned as I have not used logic chips for several years yet I seem to remember a 20ns propogation delay.

 

[Doug Owen]

I guess I don't follow the needed circuit, but if the question is how to delay the signal a bit as simple as possible, the traditional CMOS solution is often a resistor in series with and a small cap across an input. A diode can be added across the resistor to make the action asymmetrical.

At least that's the 'poor man's one shot' trick I've used a time or two.

Cheers.

Doug Owen

 

[tonyb]

If they still make a 74LS123 it may work for your appliction.

 

[LED_ASAP]

Why not just use a monostable circuit? It only takes two 2N2222, 6 resistors and one capacitor. Or if you want as simple as possible, a CMOS gate plus one resistor and one capacitor should do---just calculate your delays using f=1/[2(Pi)RC], you can get hours of delay if you want just by using a big enough RC pair.

 

[Albany Tom]

I knew you guys would come up with something, but I didn't expect four completely different workable solutions... It's been a while since I've done logic stuff, too. Back then 74ALS series was new. This is CMOS, so the resistor/capacitor trick will work well. I just have to think like CMOS...

The circuit is to code the address, and generate a negative going trigger from a pushbutton. Here's what I finally came up with:

A=1x2
B=1x3
trigger=Ax(3x4)

One quad 2-input schmitt trigger 74HC series AND, with a RC debounce into the gates. 1,2,3,4 are the pushbutton inputs, 'trigger' is the trigger output, and A and B are the binary output addresses. (4 being 11, 1 being 00)

I knew I could use a timer/counter, but wanted to keep the parts count down. The multivibrator approach is my backup plan.

Thanks!