Ideas for implementing a irreversible switch

[ScKaSx]

Hi All,

I need help designing a 'blackbox' with following characteristics. When given a TTL square-wave (+5-0V) signal as the input generates a continuous +5V output. However, the timing is important. I want the output to go high (+5V) at the rising edge of the first square wave and then stay there until a manual switch stops the square wave. To complicate matters, I vary my frequency. Any ideas? (I was considering a schmitt trigger, but am not sure how to implement it).

Cheers,
ScKaSx

 

[DrBob13]

Re: Irreversible Switch

Hi,
You can use a S-R flip flop using a pair of cross coupled NOR gates, also use a 2-pole switch, connect one pole of the switch between the reset input and Vcc to reset the flip-flop, use the other pole to feed the input squarewave through so you can interrupt it.
Regards,
Bob.

 

[echo47]

Irreversible Switch

I don't understand "irreversible switch", but if I correctly understand your timing, you can use a retriggerable one-shot such as a 74HCT123. While you apply input pulses, the output remains high. Discontinue the input pulses, and pretty soon the output will go low. Choose RC values so the timer is somewhat longer than the period of your lowest frequency square wave.

Here's a data sheet:
**broken link removed**