Mystery project - Page 1
Forum section: electronics / beginners

author: elroy
author_rank: Regular Contributor
timestamp: October 19, 2017, 09:37:48 pm
content: While rooting around in the basement today, I came across the small perfboard project shown below that I probably made sometime around 1980. It has an LM324 quad op amp chip, 16 resistors, 6 capacitors, "in," "out" and "aux" terminals, and some kind of 4-pin block on one side. (Also what appears to be a small diode hidden by that big capacitor.) It appears from the reverse side that three of the LM324's four op amps are wired-in. I see that in building it I made use of both solder and wire-wrap.I very definitely would not have had the ability to design it myself, so it would have come from something I was reading at the time -- various magazines, Forrest Mims, possibly though less likely Steve Ciarcia?Just off the top of anyone's heads, any thoughts on what kind of purpose this might have had, or how it would be used?

author: JohnnyP
author_rank: Regular Contributor
timestamp: October 21, 2017, 07:57:33 am
content: You will have to reverse engineer it.Draw the schematic and it will be easier to figure out its purpose.

author: frog
author_rank: Contributor
timestamp: October 21, 2017, 08:43:24 am
content: It's fair to assume that IN is an input and OUT is an output, you can determine empirically what the voltage gain is and the power will be in the milliwatts. The AUX output appears to have a 1M resistor and 101 (100n?) resistor and so will be a signal output, assuming a known load impedance. It's audio frequency I'd say.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: October 21, 2017, 09:59:01 am
content: My guess would be that you are making a line output level signal (aux) from a normal speaker output signal (in) and preserving the speaker output (out), I can't immediately see a 100 - 600 ohm line impedance resistor but without drawing the schematic it is a guess.An afterthought. I bet you were driving an LED bar, digital VU meter thing.

author: amyk
author_rank: Super Contributor
timestamp: October 22, 2017, 02:17:11 am
content: Behold, the power of bigclivedotcom and PhotoshopShould be easy to make a schematic of this.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 22, 2017, 01:30:35 pm
content: judging by the bunch of resistors only and an LM324, its a gain amplifier. the caps are for filtering only i guess. what else could it be? i was about to say this as 2nd post last night but i let others the chance...

author: KMoffett
author_rank: Regular Contributor
timestamp: October 23, 2017, 06:02:42 pm
content: Quote from: amyk on October 22, 2017, 02:17:11 amDOH! How many times I've reversed engineered small PCBs, and never thought of overlaying a reversed photo.Thanks Mjolinor!Ken

author: elroy
author_rank: Regular Contributor
timestamp: October 23, 2017, 08:13:10 pm
content: Quote from: amyk on October 22, 2017, 02:17:11 amWow, very cool. Without drawing out the schematic, I'm going to agree with other posters that it was pretty obviously an audio project of some kind. Maybe a preamp?

author: james_s
author_rank: Super Contributor
timestamp: October 25, 2017, 06:49:30 pm
content: Hook it up and see what it does.This is a good reason to label your projects so you remember later what they are. When I set aside a project I usually gather all the related bits into a ziplock freezer bag, label the bag with what the project is and then file it in a project box on a shelf in my lab. If it's based on a website or magazine or whatever I put a note in the bag with that info and the names of any required firmware or other files.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: October 25, 2017, 06:56:58 pm
content: Quote from: KMoffett on October 23, 2017, 06:02:42 pmHmm, it was not I.The reply with quote button is above the post, not below it for whatever reason.

author: cdev
author_rank: Super Contributor
timestamp: October 25, 2017, 08:17:36 pm
content: This is a very good habit to get into.Quote from: james_s on October 25, 2017, 06:49:30 pm