Basic capacitor delay circuit not functioning as expected - Page 1
Forum section: electronics / beginners

author: Paintap
author_rank: Newbie
timestamp: June 19, 2023, 05:22:22 am
content: In the circuit diagram pictured, I want device R to be turned on whenever the switch is on, and for a few seconds after the switch is turned off. To achieve this, I have set up a circuit with a 330uF capacitor and NPN transistors. All the resistors are 10k, and transistors T2 and T3 are configured to be inverter gates.However, the circuit isn't functioning, and no amount of research online or talking to GPT4 is helping me understand why. As far as I can tell, this circuit should work.Since my handwriting is poor and diagram undetailed, here are additional details. Maybe you can do a better job at figuring out whats going on:- 12V supply- 330uF electrolytic capacitor- NPN transistors, BC639- 10KOhm resistors- 24V LED as diode (only diode I had around when making prototype, don't think should be causing the problem, seems to work fine)

author: wasedadoc
author_rank: Super Contributor
timestamp: June 19, 2023, 07:12:22 am
content: The effective voltage change at junction of R5 and R3 is too small. Try proving that by adding a 2k7 resistor between base and emitter of T3.

author: Paintap
author_rank: Newbie
timestamp: June 19, 2023, 07:41:32 am
content: Thank you, I will give this a shot when I'm next at my desk

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 19, 2023, 08:00:24 am
content: Isn't this circuit, if working properly, double edged? i.e. It keeps the load ON for a while after you let go of the switch, but only after you keep the switch ON for a while.You may want T2 to shunt directly across C1 instead of R5+C1.

author: Paintap
author_rank: Newbie
timestamp: June 19, 2023, 08:13:25 am
content: Is this because, when you turn the switch on, the capacitor needs to discharge? You're probably correct ArdWar. In the specific context I am using this circuit, it shouldn't pose an issue, but definitely a consideration if delay causes problems. My worry with removing R5 would be the capacitor shorting through the collector and emitter of T2 when the switch is on. Would this be an issue?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 19, 2023, 08:45:06 am
content: It would help if you posted a proper schematic.From your schematic it is not clear for example whether NPN or PNP transistors are used because no arrows at all are drawn in them.Also, when you use a CAD program to draw a schematic, it is also very easy to make changes to your schematic without making a mess of it.And you probably want to make some PCB's or more complicated schematics later, so if you're into electronics It's a good idea to start learning and using such a tool anyway.I personally use KiCad. As a FOSS program it's free to use and it has no built in artificial limitations. It's also become quite mature. There are lots of other EDA suites too and you can choose any you like of course.

author: Ian.M
author_rank: Super Contributor
timestamp: June 19, 2023, 09:06:17 am
content: I generally agree with Doctorandus_P, especially about drawing the emitter arrows.However, although you do need to learn how to use them, CAD schematic editors can be quite a steep learning curve that can get in the way of expressing your ideas. There's nothing *wrong* with hand drawn schematics if you use the standard symbols, try to keep them neat, and preferably use a 2B pencil on good quality paper that will stand up to erasing mistakes, so corrections don't have to be fugly blobs!LTspice says it will work as drawn, but only if you change R5 to 100 ohms.

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 19, 2023, 09:21:12 am
content: Quote from: Paintap on June 19, 2023, 08:13:25 amYou can just put the switch where T2 is and get rid of T1, D1, R1, R5, probably R4 too if you make a Darlington output. Or just use a schmitt NOT gate, which I guess would operate much more robustly.By the way, if you just want a quick dirty sketch you can even use online schematic editor. There's a lot out there likecircuitlab.comoreverycircuit.com.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 19, 2023, 10:33:18 am
content: Also, with three connections on a transistor, you can wire them up in 6 different ways, and only one of them works properly.Especially swapping the emitter and the collector can be misleading. In such a case a transistor might sometimes "almost work". (For example with a Hfe of 10 and low breakdown voltage).

author: Paintap
author_rank: Newbie
timestamp: June 21, 2023, 07:51:56 am
content: For those who were wanting a better schematic, I took a bit of time trying to learn Kicad as Doctorandus_P suggested and an updated diagram should be attached.I am still in the process of figuring out how to run a simulation in the software, I haven't touched CAD since high-school so its essentially foreign to me. I don't have a test board, just a soldering iron and Veroboard, so prototyping is difficult without simulation.Thank you to everyone who has contributed for the inputas a beginner its nice to have a good community

author: AnalogTodd
author_rank: Regular Contributor
timestamp: June 21, 2023, 12:07:33 pm
content: I think you want the base of Q1 tied to the bottom of R3. Having it tied to the rail will keep this on all the time. From there, it's always a good idea to have something to pull down on the base of your main transistor (Q2) rather than relying solely on the base current to turn it off. Add a 10k or 100k resistor from Q2 base to ground and see how it responds.