Here is an interesting combination of two basic electronic ciruits, an oscillator operating in the audio frequency range and next the audio amplifier that will allow a speaker to be connected to the oscillator. By using the amplifier to amplify the output of the oscillator, you can obtain a loud sound that can be used for an alarm or anything else that requires a loud audio tone.
In this circuit, the emitter of a bipolar transistor is connected to a pin labeled "key". If you bring this point to ground, the oscillator turns on. Remove the ground, the oscillator turns back off. This makes the oscillator easy to control using relay contacts, another transistor, or even an old telegraph key for code practice.This type of audio oscillator is known as a twin "T" phase shift oscillator. It gets its name from the phase shift network made up of R3, R4 and C3 plus C1, C2 and R2 which shifts the phase of the signal fed back from the collector to the base by 180 degrees to make the oscillator oscillate. The audio amplifier for this project uses a type of integrated circuit or 'IC' as one of the active components. IC's contain many parts such as transistors, diodes and resistors all inside a very small package.
This IC -LM386- is designed to be a small low power audio amplifier. It's easy to use and is not too easily damaged!However, be careful not to try driving a large load like big stereo speakers with this small IC, it works great for small speakers or even headphones.
On the left is a pin-out diagram of the LM386 IC. Use this to find the correct pins to solder to. Actually, it's best to use an IC socket and not to solder directly to the pins of the IC.
You could build a second audio oscillator, and change the value of the phase shift network to change the frequency of the oscillator. Some values to try are .05uF for C1 and C2 as well as 18K for R3 and R4. That will make a lower pitched note. Just do some experimenting!
This changing field induces
a voltage in the "secondary" winding of T1. The amount of voltage depends on the turns ratio
of the windings.
If the secondary has fewer turns then the primary you have a "step down" transformer more than the primary is a "step up" transformer. In this low voltage power supply we will call for a step down to 12 volts AC.
If we used a 10:1 transformer and applied 120 volts AC to the primary we would see 12 volts AC on the secondary. (Tip) Back in the 'old days' this would be called a filament transformer and would be used to power (light up) the filaments of 12 volt vacuum tubes.
OK, from the top of T1's secondary a wire connects to the anode of diode D1. It's very important to connect to the anode as we want a positive voltage. If it got connected to the cathode we would get a negative voltage and the supply would not work. It would damage the capacitor and if the capacitor failed as a short then the fuse would blow and hopefully protect the diode and the transformer! So watching polarities is utmost important.
The diode conducts only on the positive half cycles of the alternating current so the output of the diode is positive pulses, the top half of the sinewave. Next a wire connects to the inductor L1 and then from L1 to capacitor C1.
The combination of L1 and C1 form a low pass filter. Each time a positive pulse arrives at C1 it charges up, then during the time between pulses it discharges supplying current to the load until the next pulse arrives. This filtering action is what gives a nice smooth DC output.
Next a wire connects to R1. This resistor is called a dropping resistor and is used to reduce or drop the voltage supplied to D2 the LED diode. The LED is forward biased and will light when the power supply is turned on.
If there is no load on the power supply (as in this drawing) then the LED will slowly go out when the supply is switched off due to the capacitor C1 discharging though the LED.
Depending on the size of C1 the LED could remain on for four or five seconds after the supply is turned off. Finally, a wire connects from the cathode of D2 (LED) back to the secondary of transformer T1 completing the circuit.