UM 3561 is an excellent Siren generator IC widely used in Toy applications and Surveillance systems to generate different tones. It a ROM (Read Only Memory) IC which can be operated in 2 pen cells and it functions normally down to 2.4 volts. Using only a few components, it is very easy to construct a siren generator. The IC is designed to generate sirens like Police siren, Ambulance siren, Fire brigade siren and Machine gun sound.
The IC has an inbuilt oscillator and tone selection pins. Only one external resistor and a speaker driver transistor are sufficient to make a simple siren generator circuit. Its absolute maximum ratings are :
Absolute Maximum ratings
Supply voltage Min. 2.4V Typ 3V Max 3.5V
Operating current Min – - 180 uA
Internal circuits of UM3561 - Inside the IC, there is an oscillator circuit and the frequency of oscillations is controlled by the external resistor connected to OSC 1(Pin 7) and OSC2 (Pin 8). A 220 K resistorwill give satisfactory results. The oscillations thus generated will be then transferred to a control circuit which function based on the tone selection through the connections of SEL 1(Pin 6) and SEL2 (Pin 1) . The control circuit passes the signal to an address counterand then to the ROM. The tone pulses thus generated will be available from the output pin 3. Since the sound is weak, an amplifier is necessary to get loud sound. A single NPN transistor will amplify the sound.
Pin Assignment of UM 3561
Pin 1 Tone Sel.2
Pin 2 Gnd
Pin 3 Output
Pin 4 NC- Used for testing purpose only
Pin 5 +3V
Pin 6 Tone Sel .1
Pin 7 Osc 1
Pin 8 Osc 2
By changing the pin connections of Sel.1 (Pin6) and Sel. 2(Pin 1) it is easy to change the siren tones.
The circuit works off 3 volts. If higher voltage is used, regulate the voltage to 3.1/3.3 volts using a zener diode. low voltage Zener diodes down to 1.2 volts are available now.Pin5 is the Vcc pin of IC and the zener can be connected to it.Use a Mylar speaker to make the gadget compact.
All the components are easily available in Indian market. 3.1/ 3.3 Volt zener cost Rs. 2 only. IC UM3561 cost Rs. 8 and AC187 Rs. 2.