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SN74LS139A: Input delay

Danilo A.
Mastermind 27470 points
Part Number: SN74LS139A
Other Parts Discussed in Thread: SN74HCS139

Hi Team,

Our customer would like to delay the output of SN74LS139A for about 1sec and he added an RC parts at the input. I understand that this solution is not possible. Do we have a device that could be use to delay the output? 

Regards,

Danilo

  • +1
    TI__Guru* 96947 points

    Hi Danilo,

    It is possible to get a delay that high with large capacitors, but it will not be an accurate solution as capacitors can vary significant in value and input thresholds also can vary significantly. That may not be an issue though, since it sounds like this may be for a single application where tuning can be performed. I wouldn't want to do something like this in a production product.

    For example, using the SN74HCS139 (because it has high-impedance inputs with Schmitt-trigger architecture) plus a 100kohm resistor and 10 uF capacitor should provide close to a 1s delay. The resistor value can be adjusted up to about 470k and down to 1k without any issues. If more than 470k is required to get the timing, I would recommend to increase the capacitor to keep the resistor within the 1k to 470k range.

    A more accurate and more modern solution would be to use an MCU to count out the delay -- TI offers multiple low-cost and low-power MCUs that could easily be configured for this task.