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Random timer for keyboard

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aosborn

New Member
I have interest in making a random pulse timer.

I have a keyboard interface that I've matrixed out so that's done. I want to randomly (every 10-30 seconds[or whatever]) send a pulse to the keyboard interface.

About the keyboard interface. When 2 contacts are touched a keystroke is recorded to the PC. So I just need to bridge that connection when the timer goes off.

Timer has to be randomized too as I said above.

I can do soldering and all of that, I've messed with some schematics using 555 timers. But I don't have a clue how to actually design a tiny circuit like that.
 

RadioRon

Well-Known Member
There are a few common circuits around. Here is one:

http://www.belza.cz/files1/randgen.htm

Sorry its not in English.
Here is another (see attachment).

I think both of these suggestions work on the same principle.

For the attachment, you would want to adjust the clock rate by either changing the values of C1 and R1, or tap the first 4021 at different points. Also, you should ignore the switched outputs including the LED and the 4.7K+10uF circuit. These aren't needed in your case. Be sure to attach either Vdd or GND to those INPUTS pin 7...pin 1 on the second 4021. The text says you can change the bit patterns by changing which of these pins you ground and which are to Vdd so it might be fun to put a DIP switch array here. If you don't know how, let us know and we can suggest a simple way.

(PS: attachment is from Engineer's Notebook, Integrated Circuit Applications, 1980 edition, Forrest Mims III, Radio Shack. This is a very good book for beginners as it shows many common circuits)
 

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aosborn

New Member
Wow. That seems kind of complex.

If it suits my application I can build it with a bit of help.
I just would need to know which parts to pick up from Radioshack. That schematic doesn't really make much sense to me sadly.
 

RadioRon

Well-Known Member
If that circuit is too much for you, then you should start with a kit such as this one:

http://www.qkits.com/moreinfo.cfm/QK52M

It may be easy to adapt this to supply you with pulse output by adding a simple transistor in place of one of the LEDs. The only thing I'm not clear on is how slow you can make this kit go, and if it is slow enough for your application.
 

aosborn

New Member
If I am told how to connect everything and what I need I'm sure I can make it. If anyone is willing to give me some sort of a tutorial on a basic random pulse circuit.
 
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