A time delay circuit... - Page 1
Forum section: electronics / beginners

author: Mint.
author_rank: Frequent Contributor
timestamp: May 13, 2012, 12:01:56 am
content: So I am building a little system for my home so that you can call people from one room to another, it consists of a button which when pressed charges a capacitor which then turns on a darlington pair of transistors to power a relay which stays on for 15 seconds powering a astable multivibrator circuit and then it is also supposed to power a little buzzer for about 3 seconds to grab peoples attention. My problem is that I do not know how to construct the last bit, the buzzer. I do not want it to be on for too long because it is extremely annoying! I will run the circuit on 12V.

author: IanB
author_rank: Super Contributor
timestamp: May 13, 2012, 12:10:37 am
content: What's the astable multivibrator circuit doing for those 15 seconds?I'm not quite sure about your design. Logically it would go like this:1. You press a button2. The button is connected to an electronic timer circuit that generates a 15 second pulse.3. The 15 second pulse operates a switch, such as a relay, which turns on the intercom.4. At the remote end of the intercom is an electronic timer circuit that generates a 3 second pulse.5. This three second plus activates the buzzer (a little low voltage piezo buzzer would do).Both timer circuits can be constructed using a 555 timer IC. Use the example from the data sheet that converts a rising edge to a timed output pulse.

author: Mint.
author_rank: Frequent Contributor
timestamp: May 13, 2012, 12:43:27 am
content: Forgot to mention! I do not wish to use a 555 timer because I do not understand its operation.I have attached the schematic for the circuit that I will be using, sorry for the mess.

author: vxp036000
author_rank: Regular Contributor
timestamp: May 13, 2012, 12:54:06 am
content: A 555 timer really is the simplest way to do this. Here is a good explanation of how it works:http://droolcup.com/pcomp/555.htmlI personally would use a simple microcontroller to handle all the timer functions. Overkill, yes, but it eliminates a whole lot of messy circuitry.

author: Mint.
author_rank: Frequent Contributor
timestamp: May 13, 2012, 03:54:57 am
content: Quote from: vxp036000 on May 13, 2012, 12:54:06 amBoth are too much overkill for me! I just want to build out of discrete components! I've already done 2/3 of the circuit, I just need a circuit for the buzzer, seriously I don't think it is that hard, is it?Quote from: IanB on May 13, 2012, 12:10:37 amThe astable is just flashing the LED's.Here is how it works, you're almost right1. You press a button2. The button is connected to an electronic timer circuit that generates a 15 second pulse.3. The 15 second pulse operates a switch, such as a relay, which turns on the intercom.4. The buzzer sounds for 3 seconds and then stops, however the multivibrator will be flashing for the full 15 second pulse.

author: Psi
author_rank: Super Contributor
timestamp: May 13, 2012, 04:05:49 am
content: Quote from: vxp036000 on May 13, 2012, 12:54:06 amsame.

author: IanB
author_rank: Super Contributor
timestamp: May 13, 2012, 04:15:09 am
content: Saying "use a microcontroller" to someone who is just beginning with circuits is like saying "run faster" to someone who is trailing the field in their first running race. Sure, if our aspiring athlete could magically grow wings it might be of some value, but in the real world it is likely to earn you a kick in the nuts.

author: IanB
author_rank: Super Contributor
timestamp: May 13, 2012, 04:17:47 am
content: Quote from: Mint. on May 13, 2012, 03:54:57 amNo, it's not hard. Take that astable multivibrator circuit and learn how to make a similar monostable multivibrator circuit. With a slight adjustment you can make it switch once for 3 seconds and then return to the starting state, rather than switching back and forth continuously.

author: Mint.
author_rank: Frequent Contributor
timestamp: May 13, 2012, 05:37:00 am
content: Quote from: IanB on May 13, 2012, 04:17:47 amAha! Fantastic! Thanks for the tip IanB, after a bit of research I have found a Monostable Multivibrator circuit, just tried to breadboard it, it didn't work, simulated it... it works! So now I just got to get it working on the breadboard, tweaking it a bit and I'm done!Here is the link:http://www.electronics-tutorials.ws/waveforms/monostable.html

author: SeanB
author_rank: Super Contributor
timestamp: May 13, 2012, 05:43:52 am
content: And for the buzzer reduce the capacitor values, and replace one of the LED's and resistor with a 32R speaker. For the delay use the same circuit you used for the 15 second delay, just reverse the capacitor and diode and place a base current limiting resistor of around 47k in the base lead of the darlington, driving the multivibrator from the collector..