Now that I'm a "genius", the boss thinks that I remember how to do this stuff because I did it 10-12 years ago. Any how, I digress.

The boss was to modify a piece of equipment to add a remote announciator (buzzer) with a silence button. The trick is the silence button. The only signals I have access to will be ground and the buzzer power, no constant power source (Vcc) via optocoupler or relay contact output. The button is to be momentary so no one can turn it off and forget to turn it back on.

My initial thoughts are to convert the always on output to a one-shot output to latch a relay with a NC switch to unlatch it. The relay and buzzer and can draw their power from the incoming voltage. I need a one shot circuit that doesn't require a constant power source (Vcc). So that rules out most ICs (555, etc.) and I'll need to go straight analog.

I was thinking of using a PN2907a with a RC net on the base, the emitter tied to the incoming power, and the collector tied to one side of the relay coil. The incoming signal will go through the resistor to the base and cap (the the cap connected to ground on the other side). The resistor will limit the current to the base and capacitor, making a nice RC time constant (I hope). I think a 100mSec second pulse will be enough to engage the relay. After the cap is charged, the collector will stop flowing current. Will the capacitor supply too much current to the base and blow the transistor?

If anyone has any other thoughts I'm open to suggestions.

P.S. I will try to get the schematic editor working again and see if I can post my idea.

 

Details? What's the voltage to the buzzer? How much current is available to the buzzer when it's on? Is it AC or DC? Can you disconnect the buzzer?

Ken

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"To invent, you need a good imagination and a pile of junk."
Thomas A. Edison (1847 - 1931)

 

It is all DC.

The current for the buzzer is a bit of an unknown. The buzzer is 3-28VDC, but I need to find out how loud it is at various voltages and how loud they what it to be. I'm hoping to get away with either 5 or 12VDC.

I don't know how much current the buzzer draws and I don't know how much current is available from the manufacturer's circuit directly . From the looks of the other output circuit there's enough to run some interface or isolation circuitry (which I left out of the initial description for simplicity). I was describing the circuitry down stream of the isolation circuit (a simple optocoupler). This output of the isolation circuit will come directly from the power supply (for maximum current capacity) but will switched by the current output and thus mimicking the output state and not always be on.

Disconnect the buzzer? We are trying to add the buzzer to the system, it wasn't original part of the system and we're trying to add it. I am trying to add a discconect that will automatically reconnect when the output signal goes low (no alarm).

 

if you can have the buzzer wired using DC through an SCR and a resistor in parallel. it could be triggered by any subsystem by just giving a momentary plulse at its Gate.
the buzzer turns ON.The resistor in parallel to buzzer, is suggested because, if it is self interrupting DC bell type, the SCR won't support. By having a resistor in parallel, constant current can flow and the SCR is kept ON

Now comes the silencing the buzzer.
if you momentarily short the SCR gets switched off, and wont turn on by itself , unless triggered by another sub system. Of course , you need to take care of not fusing the supply line serving the buzzer. and SCR.

Attached Thumbnails

 

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Regards,
Sarma.

 

Sarma's SCR circuit is simple and should work, but it's good practice to add a resistor in the SCR gate circuit to limit the initial gate current.

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Carl
Curmudgeon Elektroniker

 

Thanks Crutschow, You are right, OP could have a 100 Ohm resistor in series with gate before reaching the triggering elements perhaps.

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Regards,
Sarma.

 

The circuit looks great but it has one too many lines in. The output I am dealing with only has two wires, the alarm out and ground. So in the schematic from mvs sarma, I don't have +ve nor do I have a pulsed trigger event.

The voltage from the buzzer has to be derived from the power from the alarm signal which is high when in alarm and low when not.

 

Love mvs sarma's circuit. If you add an RC circuit to the gate it will provide a power-up trigger. Using a Sonalert will allow you to eliminate the resistor across the "buzzer". The 100K is there to rapidly discharge the cap when the alarms ends.

Ken

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__________________
"To invent, you need a good imagination and a pile of junk."
Thomas A. Edison (1847 - 1931)

 

What do you mean power is available "when in alarm"? In your original post you also mention "incoming voltage". It's unclear to me what power you have and why the buzzer needs to be powered by this.

As they say, a picture is worth a thousand words. Can you draw a diagram?

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Carl
Curmudgeon Elektroniker

 

Here's a block diagram of what I have and what I need.

I have power from the alarm signal, which is available when in alarm and not available when it isn't, and I have ground. I need to develop a circuit that will silence the active alarm by a push button switch. When the alarm occurs the next time (goes off and then back on) the buzzer will sound again.

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What is the significance of using a Sonalert over a different type of buzzer?

I was finally able to get my old sim software running, but I don't know if it's right. According to my simulation (in which I had to replace the buzzer with a resistor) the alarm silences itself (silence=buzzer) after the turn on delay and then refires each time the button is pressed (or in my case, when the relay fires). The buzzer needs to run until the button is pushed and not again until the next alarm.

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What I was going to say was essentially the same as ADWSystems.

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Carl
Curmudgeon Elektroniker

 

Getting the circuit software reinstalled from eons ago has been a most painful experience. But I think it's been worth it. The schematic attached got a little convoluted by all the relays I had to put in. Relay 1 and 4 are just switches. Relays 2 and 3 would be implemented as a single DPST relay (not two relays, notice the coils are wired in parallel). RBuzz is again a concession for not have a sim symbol.

Now I just need help with the values for R2, R10, and C1. I just need a time constant long enough to pull in relay2&3 to establish the NO contact on (what is shown as) relay2.

My other concern, is the voltage from C1 connected directly to Q1 base going to hurt Q1?

Now if I can swap out relays 2 and 3 for something solid state, that woud be cool too. But I'm not worried about a DPST will work.

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Wow, so complicated.
Try this:

Buzzer on NC contact of relay, it sounds when the input comes. When switch is closed, relay pulls in and latches with its NO contact. Buzzer, on NC, loses power.

When signal (power) is removed, relay unlatches.

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de KI6RWX

 

Yes. Much less complicated. Kind of a complementary solution where the buzzer is on while unlatched and latch to turn it off (which is the opposite of my thought process).

Thank you.

P.S. any ideas on swapping the relays out for something solid state?