Help with 555 delay-turning-on circuit, instant on - Page 1
Forum section: electronics / beginners

author: vv_eev
author_rank: Contributor
timestamp: July 17, 2023, 08:49:31 pm
content: Goal: Active high input and output. 3 second on-delay for the output of the 555, instant off, reset counter when the input goes low for any amount of time.I used the monostable circuit I found in datasheets, online, and which was recommended to me. I have 2 test boards, one with just the 555 circuit, and the other with more complicated sensing and solid state relay output.The plain 555 circuit times correctly if I power it directly, measuring the output with a DMM. The other board, assuming there are no soldering problems, switches instantly with no delay at all.1. Is this the correct circuit? I think it must be because the simple 555 circuit times correctly.2. What would cause this loss of delay function? I'mreading herethat I might need a cap on the reset pin, but also that this didn't work.

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 17, 2023, 10:11:50 pm
content: Quote from: vv_eev on July 17, 2023, 08:49:31 pmThe circuit won't reset if the input goes low for a short period of time. This is because CT2 will still have a charge on it, keeping pins 2&6 below 1/3 threshold when Vin goes high again and thus pin 3 of the 555 goes high immediately. There is definitely a minimum time that Vin would have to go low so that CT2 is fully discharged when Vin goes high again.It's a bit of an unconventional design in that the Vin "trigger" also powers the 555 timer circuit.EDIT: You could shorten the time it'll take for CT2 to discharge by adding a diode and resistor like this. The resistor may or may not be needed depending the impedance of Vin when it goes low.

author: vv_eev
author_rank: Contributor
timestamp: July 18, 2023, 01:04:15 pm
content: Yes, I understand it can't really be "instantly" due to the analog components. Thanks, I'll try adding that, but I still don't have a delay on the full circuit and I'm not sure why. Everything seems to test fine with a DMM including capacitance.

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 18, 2023, 02:52:37 pm
content: Post the full schematic. Hard to tell without seeing the rest of it.

author: vv_eev
author_rank: Contributor
timestamp: July 18, 2023, 03:52:42 pm
content: The rest of the circuit is pretty irrelevant. There are 2 fully built boards now. One times, the other doesn't. 5 to 10 VDC in, Q is the output which we can expect to be a diode or 2 worth of voltage lower than input on pin 8 VCC. I removed and replaced the timing passives with no change.Could it just be a bad timer chip? Are there any other common causes of losing timing functionality? Adding a cap across Gnd and VCC didn't help.

author: themadhippy
author_rank: Super Contributor
timestamp: July 18, 2023, 04:59:44 pm
content: Quoteis it? The rest of the circuit maybe interacting with your timing circuit in some overlooked way.

author: vv_eev
author_rank: Contributor
timestamp: July 19, 2023, 01:03:08 pm
content: Quite sure. If you read above, it has the same fault regardless whether it's the timing circuit alone or the rest of the board is populated. It *appears* identical now to another fully populated board that works fine. I don't measure any bad solder joints or bridges, so I'm wondering what I might be missing, without desoldering the timer which is a real pain.Components were all from Digikey (and no "marketplace" parts).

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 19, 2023, 08:49:53 pm
content: Measure the voltage on pins 2&6 of the 555 when Vin goes high. The voltage should ramp down (Start at Vin voltage and decay to zero) over a 5 second time period or so. Pin 3 should switch high when the voltage at pins 2&6 equals 1/3 the voltage of Vin.You can also test your 555 in circuit like this:Apply Vin and leave it applied. Note status of pin3; it should be high after 3 seconds. (In your case you say it's high right away)Connect a 100K resistor across CT2. Wait about 10 seconds. Measure the voltage on pin 2/6. It should have ramped up to apx 73% of Vin. (If vin = 10V it should be apx 7.3V. Vin = 5V it should be apx 3.65V. The 555 should have switched pin 3 low when the voltage reached 2/3 of Vin. If it didn't, then the 555 is bad.ie: Pin 3 should switch states when the voltage on pins 2&6 drop below 1/3 Vin and go above 2/3 Vin. If it doesn't, the 555 is bad.

author: vv_eev
author_rank: Contributor
timestamp: July 20, 2023, 01:33:01 pm
content: I think we're moving to a purely solid state timer to avoid some of the 555s quirks like the "startup bug" that needs a cap, etc. but that will make an interesting post-mortem. Thanks.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 27, 2023, 11:13:41 pm
content: Have you had look at a HC123 or similar?uP supervisor and watchdog ics have loads of crazy features (like power up reset) allways worth a look