Single pulse generation from TTL change of state

Thread Starter

chrisathome

Joined Oct 28, 2017
5
I need to generate a pulse of say 500 to 1000ms when an input changes state from TTL high (+5v) to low or vice versa. The input will be high or low continuously for hours at a time, I just need to act at the transition. The pulse will be used to momentarily operate an SPST 5v coil relay driving a usb keyboard matrix decoder to interface with software, i.e. it replicates specific keyboard keypresses. It doesn't matter if several pulses are sent at each transition, the software is tolerant.

I have to build a number of these interfaces to connect to external equipment, all of which will be acting on the same matrix decoder. Some interfaces will act on a rising edge (I.e. the input going high) and some on a falling edge. As I said before, the input lines will be either high or low for hours or even days at a time, its the state change I'm interested in. I don't care if I use TTL or CMOS (or whatever the modern equivalent is).

Its over 30 years since I did any logic work so I'm very rusty! Any help would be appreciated.

Many thanks

Christopher
 

dl324

Joined Mar 30, 2015
16,846
Welcome to AAC!
I don't care if I use TTL or CMOS (or whatever the modern equivalent is).
Use your choice of TTL or CMOS one shots (74121, CD4538, NE555, LMC555). Half a second is a bit long for using a CMOS gate with an RC network.
the input lines will be either high or low for hours or even days at a time, its the state change I'm interested in.
You'll need an edge detector circuit to trigger from rising or falling edges from the same trigger input.
 
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