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How to Add a Crystal in Electronic Siren Circuit

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How to Add a Crystal in Electronic Siren Circuit

Postby bearing » Thu Jun 08, 2017 7:50 pm

Good day,all

I have finished a electronic siren circuit which based on three transistors.Please look at the following circuit diagram:

This electronic siren circuit I found on internet is based on a complementary transistor pair consistin of Q2 & Q3 (BC557 & BC 37) wired as an astable multivibrator oscillator,which directly drives the speaker.The transistor Q1 is used to provide a full charge on capacitor C2 when power is turned ON. When push button switch S1 is pressed , the capacitor C2 slowly discharges through resistor R8.This makes the circuit to oscillate at a low frequency that increases to a high frequency and kept indefinitely as the capacitor is fully discharged. When the switch P1 is released, the output frequency decreases slowly as C2 is charged to the positive voltage through resistance R6 and the Base-Emitter junction of tramsistor Q2. When C2 is fully charged to the positive battery voltage the circuit stops oscillating.Well,I use a 12V battery to power the circuit and the switch S1 to activate the alarm.

The BC557 datasheet:

After finishing the circuit,it can work.However,I want to ask that how do I add a crystal to stabilise the frequency when battery voltage drops?

I am just interesting in this circuit,I’m not professional. So please forgive me any foolish questions!


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Re: How to Add a Crystal in Electronic Siren Circuit

Postby kb4mdz » Sun Jun 11, 2017 11:51 am

A crystal wouldn't do for stabilizing the frequency when the battery voltage drops.

To stabilize the battery voltage, you would use some sort of voltage regulator to bring the supply voltage for the oscillator circuit to something lower than the 12 v supply.

Now, the following is just off the top of my head, Maybe a 9.6 volt regulator or something lower; 7.5V, or something. Put it between the S2 on/off switch and the emitter of Q2. Don't know what that will do to the frequency range you get out of the circuit, maybe lessen the total frequency range, but that would be worth looking at on the bench.

Don't worry about the questions being foolish. "The only foolish question is the one you don't ask" Asking questions is how we learn. And even the professionals had to start somewhere.

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Re: How to Add a Crystal in Electronic Siren Circuit

Postby chartofmaryland » Tue Jun 27, 2017 10:37 am

Afternoon Bearing,

When voltage drops are causing issues in a circuit, you can typically use a capacitor to stabilize the voltage.

Not sure what the chief complaint is you are experiencing but the siren does not appear to want to draw a alot of current so a 4700uf cap across the 12vdc leads should surffice at quick voltage fluxuations. Make sure to wire the cap in with proper polarity. + to batt+ and so on

If the lights are out when you leave the station and then come on the second you key up, you know you have enough power.

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