Kitchen timer to drive a relay

Hi,
I am trying to get a small digital kitchen timer to latch a relay. The timer is powered by a 1.5v battery and emits sound via a piezo buzzer. I have removed the piezo and brought the piezo drive wire and the ground (which was wired to the piezo out. When the time out is reached the piezo receives positive pulses for about one minute. I want the first pulse to latch a SCR which then holds a relay. The supply voltage is 30V.

The problem is that the 2N3904 is not switching when the piezo pulse gets to it. I think I may have the 2N3904 biased incorrectly.

I will appreciate any help or advice.

I am sorry to bump this up, but I really need some help on getting the 2N3904 switched on by the signal (+0.8V pulses) that was used to drive the piezo buzzer.
Or should I use any other method to trigger the SCR to latch from the signal?
Your help is much appreciated!

Many SCRs wont latch from pulses. You will have to scope the pulses to be sure they will fit the scr specs.

An easier way to do this would be to use the piezo out as a trigger for a 555. When the 555 receives the pulse, it could latch the relay.

retched said:

Many SCRs wont latch from pulses. You will have to scope the pulses to be sure they will fit the scr specs.

An easier way to do this would be to use the piezo out as a trigger for a 555. When the 555 receives the pulse, it could latch the relay.

Click to expand...

retched, can you post a schematic of what you are suggesting?
Thanks

Here is a schematic that should do the trick. Some experimentation may need to be done according to the actual pulses.

A latching relay is a relay that does not require constant power to hold. It only requires a pulse to change states.

If you want, a button could be added to turn off the relay when you are done.

retch,
Thanks a lot, but I should have mentioned that my VCC is 30V as per my schematic in the first post and the relay is 24V. Can I still use a 555 successfully?

Sure, you can just use a voltage divider to get 15v and all should be fine. You will have get a relay whose coil can operate on the 15v.

If you need the 30v on your device side, just split the 30v wire and use the voltage divider on one side. That way, you will have a 30v and a 15v line.

http://www.allaboutcircuits.com/vol_1/chpt_6/1.html

http://en.wikipedia.org/wiki/Voltage_divider

Thanks, I will play around with this during the week

retched said:

Here is a schematic that should do the trick. Some experimentation may need to be done according to the actual pulses.

A latching relay is a relay that does not require constant power to hold. It only requires a pulse to change states.

If you want, a button could be added to turn off the relay when you are done.

Click to expand...

retched,
Will the piezo 0.8V positive pulses trigger the 555? I was under the impression that negative pulses trigger the 555.
For info, the positive pulses from the piezo repeat themselves for one minute once the timer times out.

Ok. A few changes will have to be made. Bill_marsden probably has a tried and true solution, but using a +.8vDC signal a triggering sub-circuit will be needed. A NPN transistor from the piezo line should do the trick.

Using an emitter follower will give a gate sig. of around .1V. To reliably trigger the SCR , should have 2V or 100μA gate drive. Since timer is floating[?], its ground may be lifted by say .3V by R1 & R2 to make input look like 1.1V peak. Add one 2N3904 as an amplifier, invert to supply a + gate signal.

How long do you want the relay energized for? Do you want a latching that you press a button to reset, or just one that holds the relay closed for the 1min length of the piezo signal, then release?

Bernard said:

Using an emitter follower will give a gate sig. of around .1V. To reliably trigger the SCR , should have 2V or 100μA gate drive. Since timer is floating[?], its ground may be lifted by say .3V by R1 & R2 to make input look like 1.1V peak. Add one 2N3904 as an amplifier, invert to supply a + gate signal.

Click to expand...

I was trying to do it with one 2N3904 and getting stuck.
Thanks will try that this coming week, much appreciated.

retched said:

How long do you want the relay energized for? Do you want a latching that you press a button to reset, or just one that holds the relay closed for the 1min length of the piezo signal, then release?

Click to expand...

Yes I want it to latch a relay permanently and will release it by throwing a switch. Basically it needs to latch when the first 'peep' from the piezo is received. In the diagram I posted it latches properly but I just could not get the 2N3904 to trigger the SCR and having it latch when the pulse is received

Apologies for responding late, but for some reason this thread became "unsubscribed" and I missed your replies when you did them.

Ah. How long are you talking about being latched?

A minute? hour? day? year and a half?

If it is more than a few minutes, I would get a latching relay.

Latching relays change states with a pulse, then use no power at all to hold state.

So the piezo pulse can set the relay, then you can use a pushbutton to pulse it again.

If it is a short period of time, a standard relay will do. Use a NO (Normally Open) relay so the piezo trigger will close the circuit, then when you hit the power switch, it will de-energize, returning the relay to its NO state.

retched said:

Ah. How long are you talking about being latched?

A minute? hour? day? year and a half?

If it is more than a few minutes, I would get a latching relay.

Latching relays change states with a pulse, then use no power at all to hold state.

So the piezo pulse can set the relay, then you can use a pushbutton to pulse it again.

If it is a short period of time, a standard relay will do. Use a NO (Normally Open) relay so the piezo trigger will close the circuit, then when you hit the power switch, it will de-energize, returning the relay to its NO state.

Click to expand...

The relay wil latch at the end of a timed period which could be anything from a few minutes to a few hours and then stay latched until I manually switch the device off. That period again could be anything from a few minutes to a couple of hours depending if I am close or not.

Will a latched relay ignore the repeated pulses from the piezo or will it change state continuously with each pulse?

I have not been able to find any latched relays here from our suppliers. Do you have any more details or a link to them?

You will use a 555 to send the pulse that is triggered from the piezo. The 555 mono will only send 1 pulse, ignoring the rest.

www.control-logic.com.au/persistent/catalogue_files/products/HC3.pdf

http://en.wikipedia.org/wiki/Relay#Latching_relay

I cannot find any suppliers for any type of latching relay here and to import one may just be too expensive.

Maybe I need to rethink what I was trying to do and that was to disconnect a very low current path (30V line <20mA) when the timer times out by means of a relay contact and then latching it out until reset by hand.

If I throw out the relay then I need to use a semiconductor to disconnect and latch off when the piezo chimes and only be resetable to on by pushing a button.

How would one make a transistor switch +30V through by pushing a button and then switch off and stay latched off from the piezo pulse and reset to on by another button?

Do you have any suggestions in that direction?

If it was a problem with the pulses -
Put the pulses through a diode with a capacitor to collect charge as a path to ground and have the trigger connected on the top of the Capacitor so that when it collects enough voltage it will trigger.

I would worry that your 1.5 volt timer is not throwing enough voltage at the SCR, so wind yourself a little autotransformer to send those pulses through the center tap and use a diode and capacitor as suggested before but on the up converted voltage at the top of the transformer. Efficiency should not be critical so you should seriously have no trouble making your little autotransformer out of scraps of enamel wire pulled from scrap equipment.

Suzukiman said:

Yes I want it to latch a relay permanently and will release it by throwing a switch. Basically it needs to latch when the first 'peep' from the piezo is received. In the diagram I posted it latches properly but I just could not get the 2N3904 to trigger the SCR and having it latch when the pulse is received

Click to expand...

You can't do it with a single 2N3904.

Either you use the circuit that was suggested by Bernard or you try mine, show below. One word of caution though. The 2N3904/06 are rated for 40V only and I would not feel comfortable using it at 30V. I usually uses transistors that rate over 100V in these situation.

To release the relay, you can place a push button across the SCR(A & K) terminal.

Note: Missed a resistor in Base of 2N3906. Thanks to Potato Pudding for pointing it out in post#21.