A very simple electronic horn that require few external components and is very easy to construct can be build using this circuit diagram . When the K switch is closed the oscillator formed by transistor T1 and T1 will generate a rectangular signal . This electronic horn circuit will consume a maximum output power around 40 watts , so the audio signal of this horn circuit will be a little loud . This horn circuit require a 6 volts DC power supply . The T1 , T2 transistors must be a BC107 type or similar . The T3 transistor should be a 2N2613 npn transistor or BC 140,141, 2N1990, 2N2102, 2N2405 or other similar type . T4 transistor must be an AU106 PNP transistor or AU 113, AU 213, 2N5324 or other similar type .
The D1 diode that is used for protections of the T4 transistor must be an 1N5400 ,1N5401 ,1N5408 or other similar type .
A very simple electronic thermostat circuit can be constructed using this electronic circuit that can be used for controlling a thermal resistor a fan or other device that is dependent by temperature .
The temperature which torn on or off the controlled is set using a 50kohms potentiometer .
When the temperature is low than the preset temperature , the T2 transistor conducts and the relay activate the thermo-element (thermal resistor ) or if at the relay terminals is connected a fan , the circuit will turn off the fan .
The T1 transistor must be a npn type BCY58 type ( or similar ) and T2 transistor must be a pnp type BCY78 ( or similar type ) . The Th thermistor must be mounted in that place that needs a constant temperature ( controlled temperature ) . This electronic thermostat circuit needs to be powered using a 24 volts DC power supply , so you need to use a 24 volts relay .
The thermistor used in this project has a value of 5k ohms .
A very nice alarm circuit can be constructed using one thyristor (SCR) a buzzer and other few external components . This alarm circuit can work in parallel loop or closed loop configuration . As you can see in this electronic schematic circuits the device is very simple . For the parallel loop configuration is used four switches connected in parallel and are used for monitoring . When one of the four switch is closed SCR1 triggers , which sound the alarm .
The function of the closed loop circuit alarm is same as parallel loop alarm , but in this case the alarm is activated when one of the switch ( that is connected in series at this alarm type ) is open .
This alarm circuits ( parallel loop and closed loop ) need to be powered using a power supply that will provide and output DC voltage between 6 and 12 volts .