RC Delay circuits  - Page 1
Forum section: electronics / beginners

author: l0rd_hex
author_rank: Regular Contributor
timestamp: July 19, 2022, 12:45:29 am
content: Hi friends,I'm trying to design a circuit with some specific constraints, specifically the circuit is powered by 5V DC the entire time and I want to drive a 2SC3225 (NPN transistor) for a few seconds and then stop. I've been trying to simulate this circuit out in easyeda but my circuit design knowledge is pretty limited.From what I understand, I'm probably looking at a RC circuit but I can't seem to get my head wrapped around how charging up a cap is going to stop something (another transistor?) from driving the 2SC3225.I'd like, if possible to do this with just resisters, capacitors and transistors but I could also use a 555 if needed (another related project has a 555 which I could switch out for a 556 if needed).Here's a timeline based approach of what I'm trying to do:0 seconds: 5V DC starts, 2SC3225 is activated3 seconds: 5V DC continues, 2SC3225 is activated5-10 seconds: 5V DC continues, 2SC3225 is turned off11 - forever: 5V DC continues, 2SC3225 stays offI hope that explains what I'm trying to accomplish, this is one of those, I don't know the language to address the problem sorta things.Thanks!

author: themadhippy
author_rank: Super Contributor
timestamp: July 19, 2022, 01:45:05 am
content: try doing the opposite, transistor stays off for the first 5 seconds then turns on,then chuck in another transistor as an inverter

author: Ian.M
author_rank: Super Contributor
timestamp: July 19, 2022, 03:58:38 am
content: Here's one way of doing that.Q1, Q2 act as a high gain amplifier with positive feedback to provide hysteresis for snap action switching. The ratio of R3 to R4 + the Vbe drop of Q1 sets the switching point, chosen to be approx. 2/3 of the supply voltage. R3 also sets the maximum base current for Q2.As C1 charges the voltage at the base of Q1 also rises till it no longer has enough Vbe to stay on. As it starts to turn off, Q2 also starts to turn off, so its collector voltage rises, and R5 provides positive feedback to Q1's base, 'snapping' them both off. The ratio of R2 to R5 sets the hysteresis range. R6 shunts a little bit of current away from the base of Q2, handling any off-state leakage from Q1 and internal C-B leakage in Q2, and sharpening and speeding up Q2's turnoff.Q1 can be any small signal PNP with reasonably high gain.Q2 can be any small signal NPN with reasonably high gain.Q3 is in place of your 2SC3225, as LTspice doesn't have a model for your transistor. Like the 2SC3225 its a 2A part.Note that to avoid 'runt' timing periods if the 5V is briefly interrupted, you may need a diode across R1, cathode positive, to rapidly discharge the timing capacitor when the 5V is switched off.

author: ledtester
author_rank: Super Contributor
timestamp: July 19, 2022, 05:03:53 am
content: Quote from: l0rd_hex on July 19, 2022, 12:45:29 amLook up "monostable multivibrator" also known as a "one-shot". A definition fromengineeringnotes.com:QuoteThere are a multitude of ways to create a one-shot:- using TTL/CMOS timer chips, e.g. CD4047, CD4538, 74x121, 74x122, etc.- using Schmitt-trigger inverters or other logic gates- using a 555- using an op-amp or comparator- using discrete componentsSome links:-http://www.n5dux.com/ham/files/pdf/Working%20With%20Monostable%20Multivibrators.pdf-https://www.electronics-tutorials.ws/waveforms/555_timer.html

author: Infraviolet
author_rank: Super Contributor
timestamp: July 30, 2022, 12:46:14 am
content: The other alternative is using this as an opportunity to learn about microcontrollers and use an arduino or something similar as a programmable delay, this is extremely overengineered for just giving a simple delay of this sort, though can have very accurate timing accuracy, but it does then give scope for expanding the project to do mroe in later revisions. I'd probably go for the 555 option if I knew I wasn't going to want to add fancier features later on.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 19, 2022, 01:59:22 pm
content: At t=0 5V is on and stays on- it doesnt change?So you need to start a timer at t+3 and turn it off at t+5You need a sequential timer- a pair of 555s or a dual 555Here is a single chip (CD4093) way of solving this.

author: Benta
author_rank: Super Contributor
timestamp: August 19, 2022, 03:55:16 pm
content: Quote from: l0rd_hex on July 19, 2022, 12:45:29 amCompressing that, what I read is:t=0: +5 V turns on, transistor turns on.t=5...10: transistor turns off and stays off.I've no idea why you include t=3 and t=11. Please explain.Cheers.

author: MrAl
author_rank: Super Contributor
timestamp: August 19, 2022, 05:51:21 pm
content: Quote from: l0rd_hex on July 19, 2022, 12:45:29 amHello,If you dont need super clean switching action the simplest possible circuit to do this with that NPN transistor would be to place a large value capacitor in series with a resistor and connect the other end of the resistor to the base of the transistor, and the other end of the cap to +5 volts. You also need a larger base emitter resistor maybe 10k.You may also need a diode in parallel to the capacitor so then when the power shuts down the diode discharges the capacitor.Since the cap will be of large value, it will be an electrolytic type, and so the positive terminal goes to +5 volts.The diode cathode also goes to +5v and the anode goes to the cap minus terminal.This way this works is as the 5 volt power turns on the capacitor conducts current through the resistor and base emitter, which turns the transistor on. As the cap charges the base emitter gets less and less current until it can no longer keep the transistor turned on and then it start to come out of saturation so it starts to turn off. Soon after it turns off completely. Because you are only driving bulbs this should work, as long as they are not large bulbs that draw a lot of current. The initial current as the bulb starts to light up should be less than the transistor rating.You will need a rather large capacitor though not sure if you have that around, but if you add a second transistor you can use a smaller value capacitor. You would have the second transistor emitter drive the base of the first transistor and the cap and resistor connected to the base of the second transistor. Doing this means less current through the cap to run on the first transistor and so it says on longer with a small value capacitor.It all depends on what specifications you need. If you dont need super clean switching and your load does not draw excessive current it should give you some results. I cant think of anything that could be simpler and with less parts than that unless you are familiar with microcontrollers. Using an MCU you can tune the timing any way you want and besides the chip you would need just one resistor to drive the base. However, you also have to have a way to program it unless you can get someone to program it for you.You really should tell more about what you need though and show the bulbs and whatever else you have.