I built a thing.. and it didn't work! - Page 1
Forum section: electronics / beginners

author: mindcrime
author_rank: Unknown
timestamp: August 12, 2019, 03:03:33 pm
content: Dave always says he hopes our projects fail (at first), and as luck would have it, I had one of those good "learning experiences" this weekend. I had prototyped a simple little 555 timer on the breadboard, which is intended to be the clock signal for a Z80 CPU. I wanted to move it to something more permanent and make it adjustable, so I grabbed two 50K pots, the 555, the cap and some jumpers, a piece of protoboard, and started soldering!At the end, I plugged everything in, and.. crickets. Nothing happened.And so I got to spend an hour debugging... continuity tests, resistance measurements, voltage measurements, probing different parts of the circuit, etc., etc.And in the end, I measured the resistance between two pins (2 and 7, IIRC) and expected it to be around 45K, and got 90K instead. Aaahaha. Something wasn't right around one of those two connections. And finally, with enough focused physical inspection, I found a wire that looked a bit suspicious. It was soldered in place, but it appeared to both *not* be in contact with the pin it was supposed to be tied to, and possibly had a solder bridge to the wire beside it. Uuugh.So I yanked that out, temporarily jumper clipped it to the appropriate pin, and LET THERE BE LIGHT (blinken LED's anyway).The moral of this story... Heck if I know.But debugging a circuit that doesn't work is definitely an interesting experience because it forces you to think through everything, trace every part of the circuit, understand what the voltage / resistance / etc. is supposed to be at different points, etc. So maybe Dave is right to hope that things fail at first.

author: dcbrown73
author_rank: Regular Contributor
timestamp: August 12, 2019, 03:57:43 pm
content: I had a similar issue / learning experience which is detailed inthis thread.Basically, I had a voltage divider of 100k and 22k. After struggling with it not working at all, I measured the 100k resistor in circuit, but instead of 100k, I was getting a mega ohm reading. I relocated the voltage divider to a different area, and bang. It started working perfectly.I haven't the slightest as to why it was showing that high of resistance. I was measuring the leads of the resistor even though it was still in circuit.Maybe someone can help explain what was possibly occurring there. I was thinking maybe it was the breadboard, but I was testing the resistors leads. If it was trying to take an alternate path, shouldn't the 100k resistor have been the most likely path if the other path was in the mega ohm realm?

author: soldar
author_rank: Super Contributor
timestamp: August 12, 2019, 05:53:45 pm
content: The moral of the story is to always go slowly and patiently checking and double checking everything as you go. You will still make mistakes but much fewer and easier to troubleshoot.Most of the times someone posts "I built this schematic and it's not working" the answer is "No, you built something else and you need to double check everything".

author: Nominal Animal
author_rank: Super Contributor
timestamp: August 12, 2019, 07:47:28 pm
content: Quote from: soldar on August 12, 2019, 05:53:45 pmThat's exactly what I tell those I'm helping learn programming, too.Don't just put everything together, and when it does not work, start looking for the problems. It is like looking for a bent needle in a bucket of needles. With your bare hands.If you do one testable/checkable/verifiable (English hard!) part at a time, you limit the number and type of possible errors; you limit the region you need to examine to find each error as they occur. And they will, no matter how advanced you are. Doing development part by part leads to least amount of frustration.

author: KL27x
author_rank: Super Contributor
timestamp: August 12, 2019, 08:14:05 pm
content: Quotedcbrown: resistance test is meaningless if the circuit is under power. This is the only way I can imagine you got that false reading.