Doorbell Signal to Turn on 12v Relay help please! - Page 1
Forum section: electronics / beginners

author: zastil
author_rank: Newbie
timestamp: August 01, 2012, 06:58:09 pm
content: I have a 555 timer circuit that allows on/off option to turn on a relay which is working fine by connecting the 2 wires coming from the 555 each time turns on and off all is okay even using a 9v battery to power the 12v relay.My problem is that I want to be able to use a cheap doorbell I got to send the 3v signal to a transistor to allow the 2 wires coming from the 555 so when doorbell is pushed the 3v maybe less is sent from the device to the transistor to allow the 2 wires to connect.I will take some photos to show you this on a breadboard working and what trying to add!I hope you can solve this for me as I can get it working sending the 9v to the transistor to send it on/off but the 3v seems todo nothing.WayneImages are athttps://plus.google.com/photos/108816590701606354526/albums/5771786186820109713

author: KTP
author_rank: Frequent Contributor
timestamp: August 01, 2012, 07:19:37 pm
content: Am I the only one a bit confused here?Can you draw a crude schematic of what you have right now? And the part number of the transistor...

author: zastil
author_rank: Newbie
timestamp: August 01, 2012, 07:22:55 pm
content: Transistor is BC557 (pNp) I also have BC547 (nPn)

author: zastil
author_rank: Newbie
timestamp: August 01, 2012, 07:25:23 pm
content: is the 555 on/off circuitRelay Toggle Circuit Using a 555 TimerThis 555 timer circuit below toggles a relay when a button is pressed. Pins 2 and 6, the threshold and trigger inputs, are held at 1/2 the supply voltage by the two 10K resistors. When the output is high, the capacitor charges through the 100K resistor, and discharges when the output is low. When the button is pressed, the capacitor voltage is applied to pins 2 and 6 which causes the output to change to the opposite state. When the button is released, the capacitor will charge or discharge to the new level at the output (pin 3). The parts are not critical, the resistors can be somewhat higher or lower, but the 2 resistors at pins 2 and 6 should be equal values, and the resistor connected to the cap should be 10 times greater or more.Advantages of this circuit are the large hystersis range at the input which avoids false triggering, and only a few parts are needed for construction. One disadvantage is the relay may be engaged when power is first applied. To solve this problem, you could tie the reset line (pin 4) to another resistor/capacitor combination with the capacitor at ground and the resistor at the +V point. This will cause pin 4 to be held near ground for a short period which will reset the output when power is applied.The 100 ohm resistor and 100uF capacitor serve to filter noise on the supply line if the circuit is used in a automotive application. They may not be necessary. The circuit may work well without those parts.

author: Chet T16
author_rank: Frequent Contributor
timestamp: August 01, 2012, 07:50:37 pm
content: So you're looking to replace the switch in that circuit with a transistor that is turned on by a 3v output on the doorbell when activated?

author: zastil
author_rank: Newbie
timestamp: August 01, 2012, 09:48:37 pm
content: Yes exactly that!

author: KTP
author_rank: Frequent Contributor
timestamp: August 01, 2012, 10:25:39 pm
content: It is maybe not so easy to do what you want with one transistor (at least I am too sleepy to see the obvious simple solution). You have the 555 wired as a simple state machine that toggles the state based on a input action (pressing the switch). The 555 is more like a S R flipflop...if it were a JK flipflop this would be easy...just turn it into a T flipflop.Let me think on this a second. We do have that unused discharge mosfet on pin 7....

author: zastil
author_rank: Newbie
timestamp: August 02, 2012, 09:55:52 am
content: wow thanks for looking at this for me. I thought transistors were supposed to take a small current to release a stronger one, but I am very new to them.

author: T4P
author_rank: Super Contributor
timestamp: August 02, 2012, 01:10:42 pm
content: Quote from: zastil on August 02, 2012, 09:55:52 amRelease? No not release. Magic doesn't happen in electronicsThey act as switches.

author: KTP
author_rank: Frequent Contributor
timestamp: August 02, 2012, 01:24:30 pm
content: Quote from: Electroalek on August 02, 2012, 01:05:51 pmHow will that solution ever change the state from low output to high output?

author: Monkeh
author_rank: Super Contributor
timestamp: August 02, 2012, 01:35:08 pm
content: Quote from: Electroalek on August 02, 2012, 01:05:51 pmYou mean the worst schematic you've ever taken off the internet, badly modified, and stripped the original author's name from then posted as your own.

author: Monkeh
author_rank: Super Contributor
timestamp: August 02, 2012, 01:39:48 pm
content: There's plenty of room.

author: KTP
author_rank: Frequent Contributor
timestamp: August 02, 2012, 02:03:24 pm
content: I repeat. What happens in the case where the 555 output is low such that the capacitor is discharged. Activating the transistor with 3V will not pull pin 2 and 6 low to change the 555 output to high as desired.You need an analog switch or a buffer with an enable line.

author: T4P
author_rank: Super Contributor
timestamp: August 02, 2012, 02:23:34 pm
content: Well at least include the actual author itself next time

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 02, 2012, 03:08:45 pm
content: Quote from: KTP on August 02, 2012, 02:03:24 pmSince the emittor is at 6 volts nothing will happen either way with the base at 3 volts...

author: G7PSK
author_rank: Super Contributor
timestamp: August 02, 2012, 03:17:48 pm
content: Where on the door bell board do you propose to get this 3 volts from, the speaker?.If you have 3 volts why not have a 3 volt relay, there are lots of 5 volt rated ones and they will mostly work down to 3 volts or less in the small sizes. There would then be no need for the 555.

author: KTP
author_rank: Frequent Contributor
timestamp: August 02, 2012, 03:58:20 pm
content: The OP wants a 3V controlled t flipflop, which can't be made from a 555 without extra parts.I like the suggestion of using a relay or some other circuit.

author: zastil
author_rank: Newbie
timestamp: August 02, 2012, 06:14:49 pm
content: So in all I would need 2 relays? 12v/240v and 3v to 4v ugh so why are transistors used.Also to comment it's click on click off. Like a doorbell signal is pushed once to turn on and keep on till doorbell pushed again so goes off. Why I need that 555 circuit to handle it. Although have seen 2 x transistor model todo this I am not fan of transistors as part numbers are not clear enough!Also the circuit posted with 1 or 3k resistor from 3v supply still did not work! Maybe I got the wrong transistors but how do I know which one to use again damn finding out what transistor is one to use.Also I get 5v passing through the transistor and when push the doorbell goes to 3v go figure! Maybe just this to expert for me to figure out been over a week now so think have to give up and go turn on my outside lights with a switch and go outside rather than being able todo it with this doorbell signal!After watching many youtube videos on transistors from what I learned seems they can't do what I want or what was told.

author: KTP
author_rank: Frequent Contributor
timestamp: August 02, 2012, 06:36:43 pm
content: Quote from: zastil on August 02, 2012, 06:14:49 pmtransistors can do everything, it just takes more of them than maybe you want to use, so you might be better off looking at a different circuit.You could build a t-flipflop from transistors for example, then use this as a simple state machine with the output of your 240V relay and the input from your doorbell switch. It is a lot of parts IIRC.

author: G7PSK
author_rank: Super Contributor
timestamp: August 02, 2012, 08:37:19 pm
content: Just one self latching relay, forget 555 and transistors just use the 3 volts to work the relay there are relays that latch available of the shelf.

author: KTP
author_rank: Frequent Contributor
timestamp: August 02, 2012, 08:45:14 pm
content: Quote from: G7PSK on August 02, 2012, 08:37:19 pmI don't think a latching relay by itself will do what the OP wants done.On the first application of a 3V pulse, he wants the relay to latch. Ok, easy.But then the next time a 3V pulse comes, he wants the relay to unlatch. This is a toggle type flipflop machine...can't be done with one latching relay.There are some papers on doing this with 2 or 3 relays, some diodes and capacitors...You can get jk or t flipflops in 74 series dip packages, add a output boost transistor before the 120v relay, and have this circuit done in about 5 minutes.

author: zastil
author_rank: Newbie
timestamp: August 02, 2012, 09:04:15 pm
content: Yeah I am surprised this is hard todo. Is it me designing something that cannot be done? I thought electronics was awesome after watching all EEVblog but guess need more tools or components.If you think what I want done a 555, a relay and 2 transistors plus cap and some resistors you should be able todo this. Can't understand why I can't grrrrrr

author: TerminalJack505
author_rank: Super Contributor
timestamp: August 03, 2012, 01:20:14 am
content: If you really need a solution that uses the existing 555-based circuitanduses just transistors to replace the switch and hook up to the doorbell's output then here's a solution.It's basically a discrete tri-state buffer. TheEninput would hook up to you 3V doorbell signal. (Which is assumed to be 0V when off, BTW.) TheDinput would come from where the switch's left hand side currently is. TheQoutput would go to where the switch's right hand side currently is. Everything in the dotted box would replace the switch. Everything outside of the dotted box is for simulation purposes.

author: IanB
author_rank: Super Contributor
timestamp: August 03, 2012, 05:29:37 am
content: Quote from: zastil on August 01, 2012, 06:58:09 pmDo you need both the doorbell and the 555 on/off circuit, or just the 555 circuit? Because the simplest and most logical solution is to use the doorbell push button as the on/off switch connected to the 555 and to disconnect the rest of the door bell.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 03, 2012, 09:20:34 am
content: Quote from: IanB on August 03, 2012, 05:29:37 amLet me guess, it's a wireless doorbell