Electronics > Beginners

Can You Tell Me What This Circuit Does?

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kilimar:

There are no markings on the back of the board.

I looked up the chip, CD74HCT123E (https://www.mouser.com/ProductDetail/Texas-Instruments/CD74HCT123E?qs=aZHVzqKrScBW3BonhzIwjw%3D%3D)

And looked at the datasheet, but I don't have enough electronics knowledge to understand what the chip does....

Thanks!

-k

amspire:
It is a dual monostable multivibrator. That means it is a circuit that outputs a pulse of a fixed length when the input goes high (or low depending on which input you use).

The circuit seems to use 100K resistors and 22uF caps, so the pulse width is 0.45 x 100K x 22uF = 1 second. There are two of these circuits and it could be that they are connected in series.

If the circuit recieves a constant steam of pulses in at a rate of more then 1 second, it keeps retriggering and the output stays high.

The circuit could be used for extending the width of very narrow pulses, or it could be used for generating a 1 second pulse on detecting a single slow input change.

Just as an example, it could be that on applying power to a microprocessor, you want to keep it in reset mode for 1 second until the power has stabilised.

kilimar:
So what would it be used for in a standalone application?  I don't have any other electronics that this came from and I'm confused as to what it could of been used for....

amspire:
Sometimes circuits like this are made to fix a power up problem in a particular product. Something goes crazy on power-up so this board was added to allow things to start up in a sequential way. If the two monostables are in series, then the second one may output a 1 second pulse when the first pulse ends.

Unless you have a need for a something like this, it is pretty useless. This circuit could have been designed for anything - perhaps an audio power amplifier and someone wants the speakers disconnected until the power amplifier is properly powered up.

You can turn a circuit like this into a tachometer - measuring the rotational speed of a mechanical device.

Lets say you want to measure a car engine's speed - say a 4 cylinder car that goes to 6000rpm.

The maximum spark rate is 6000 x 4 /60 = 400 times per second. If you set the monostable to put a pulsewidth of 2 milliseconds and set up an input circuit so that the monstable gets a pulse on every ignition spark, then you get a 2 millisecond pulse every time there is an ignition spark.

If you measure the average DC output of the monostable, the voltage will be proportional to the engine speed.



kilimar:
That's the reason that I'm very confused that I have this board.  I haven't taken anything apart that would seemed to of need something like this.  Especially on a board by itself.  Nor do I recall taking anything apart that has an 1" x 1" board as an add-on board. 

Solar Pathway lights?
Power tool battery packs?
Laptop battery packs?
Wall chargers (USB)?
DJ Phantom battery pack?

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