Can You Tell Me What This Circuit Does? - Page 1
Forum section: electronics / beginners

author: kilimar
author_rank: Newbie
timestamp: October 07, 2019, 12:30:45 am
content: 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

author: amspire
author_rank: Super Contributor
timestamp: October 07, 2019, 01:32:25 am
content: 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.

author: kilimar
author_rank: Newbie
timestamp: October 07, 2019, 01:43:31 am
content: 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....

author: amspire
author_rank: Super Contributor
timestamp: October 07, 2019, 02:01:00 am
content: 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.

author: kilimar
author_rank: Newbie
timestamp: October 07, 2019, 05:37:24 am
content: 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?

author: amspire
author_rank: Super Contributor
timestamp: October 07, 2019, 06:16:51 am
content: 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.

author: L_Euler
author_rank: Regular Contributor
timestamp: October 07, 2019, 11:44:58 am
content: 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.

author: kilimar
author_rank: Newbie
timestamp: October 07, 2019, 04:47:57 pm
content: Interesting!

author: ozcar
author_rank: Frequent Contributor
timestamp: October 07, 2019, 05:51:56 pm
content: 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.