Author Topic: Can You Tell Me What This Circuit Does?  (Read 894 times)

0 Members and 1 Guest are viewing this topic.

Offline kilimarTopic starter

  • Newbie
  • Posts: 6
  • Country: us
Can You Tell Me What This Circuit Does?
« on: October 07, 2019, 12:30:45 am »
849848-0
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
« Last Edit: October 07, 2019, 12:32:40 am by kilimar »
 

Offline amspire

  • Super Contributor
  • ***
  • Posts: 3802
  • Country: au
Re: Can You Tell Me What This Circuit Does?
« Reply #1 on: October 07, 2019, 01:32:25 am »
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.
 

Offline kilimarTopic starter

  • Newbie
  • Posts: 6
  • Country: us
Re: Can You Tell Me What This Circuit Does?
« Reply #2 on: October 07, 2019, 01:43:31 am »
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....
 

Offline amspire

  • Super Contributor
  • ***
  • Posts: 3802
  • Country: au
Re: Can You Tell Me What This Circuit Does?
« Reply #3 on: October 07, 2019, 02:01:00 am »
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.



 

Offline kilimarTopic starter

  • Newbie
  • Posts: 6
  • Country: us
Re: Can You Tell Me What This Circuit Does?
« Reply #4 on: October 07, 2019, 05:37:24 am »
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?
 

Offline amspire

  • Super Contributor
  • ***
  • Posts: 3802
  • Country: au
Re: Can You Tell Me What This Circuit Does?
« Reply #5 on: October 07, 2019, 06:16:51 am »
It wouldn't be in a battery pack. The board is a sign of a design mistake - usually anything like this will be on the main circuit board.

Chargers may want to have a delay before they start to charge so it is not trying to charge while you are still connecting the battery, but I wouldn't expect to see something like this in a modern USB wall charger.

Looks like the IC is from 1999 and that is more like the era I would expect for a design fix like this. It may be some old appliance you pulled apart years ago.

If it is not a design fix, then the only reason I can think of for making a board like this is that they needed boards of different delays for different models. I am think of something made in fairly low quantity.
« Last Edit: October 07, 2019, 06:22:45 am by amspire »
 

Offline L_Euler

  • Regular Contributor
  • *
  • Posts: 86
  • Country: us
Re: Can You Tell Me What This Circuit Does?
« Reply #6 on: October 07, 2019, 11:44:58 am »
Many years ago I had a project to create a automatic telephone pulse dialer, presumably for an alarm system.  Telco specs for the dial pulses are 10Hz pulse at a 60-40 pulse width.  So, I first made a very accurate 10HZ square wave that fed a monostable circuit similar to yours that then reshaped the 50-50 SW to 60-40.
There's no point to getting old if you don't have stories.
 

Offline kilimarTopic starter

  • Newbie
  • Posts: 6
  • Country: us
Re: Can You Tell Me What This Circuit Does?
« Reply #7 on: October 07, 2019, 04:47:57 pm »
Interesting!
 

Offline ozcar

  • Frequent Contributor
  • **
  • Posts: 320
  • Country: au
Re: Can You Tell Me What This Circuit Does?
« Reply #8 on: October 07, 2019, 05:51:56 pm »
For years, nay decades really, I had a pulse generator using the original TTL 74123. “Mark” and “space” were set independently using a bank of switched capacitors and a pot for each.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf