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Design by BERNIE GILCHRIST Build A Siren Sound Generator This little circuit provides three siren sounds – Po­lice, Fire Engine & Ambulance. By making a simple modification, it can also be made to produce a sound similar to a machine gun. It is powered from a single AA size 1.5V cell & is ideal for games & models. At the heart of this project is the UM3561A, an LSI (large scale integration) device which includes a 256 x 8-bit ROM (read only memory) programmed to simulate the siren sounds via an internal tone generator and control circuitry. The only external component that affects the pitch and timing of the siren sounds is the resistor connected between the OSC1 and OSC2 terminals (pins 7 and 8) of the IC and we’ve shown its value as 330kΩ. Reducing the value of this resistor will increase the pitch of the siren sound while increasing the resis­tor will have the opposite effect (ie, lower the pitch). 64  Silicon Chip Varia­tion in the supply voltage to the IC also affects the pitch and timing but to a much smaller degree. Pin 6 on the IC (SEL1) is used to select the siren sound and this is achieved by switching it high (Fire), low (Ambulance) or open circuit (Police) with 3-position slide switch S2. TABLE 1 Ext. Supply R1 R2 3V 560W link 6V 3.3kW 10W 1W 9V 6.8kW 22W 1W 12V 8.2kW 27W 5W The output signal from the IC is a modulated pulse waveform which is roughly equal to the supply voltage in amplitude; ie, slightly less than 1.5V peak-to-peak. The output of the IC drives transistors Q1 and Q2 which operate as a Darlington tran­sistor to drive the 8Ω loudspeaker. The current drain through the speaker is limited by resistor R2. The value of R2 is a compromise between loudness and bat­ tery life. A lower value of resistor will make it louder but the battery life will be shorter. If you are using just the 1.5V cell as shown on the circuit and in the photos, you can replace R2 with a wire link. This makes the speaker quite loud but battery life will be relatively short. For an AA cell, we would expect the battery life to be no more than an hour or so. The 100µF electrolytic capacitor connected between Vdd (pin 5) and Vss (pin 2) is used to decouple the IC from the supply to the output stage. This prevents the relatively high Fig.1: the circuit is based on a UM3561A LSI chip. This includes a 256 x 8-bit ROM which is programmed to simulate siren sounds via an internal tone generator & control circuit. S2 selects between fire, police & ambulance sounds, while the 330kΩ resistor between pins 6 & 7 controls the pitch. S1 OFF D1 1N4007 ON R2 SEE TEXT 330k EXTERNAL BATTERY R1 SEE TEXT 1k 5 1.5V FIRE A LED1 GREEN 100 50VW  S2 POLICE AMBULANCE 6 7 8 VDD OSC1 OSC2 SEL1 IC1 UM3561A SEL2 VSS 1 2 OUT Q1 BC549 3 B 8 SPEAKER C E Q2 BC337 B C E K B A E C VIEWED FROM BELOW K SIREN GENERATOR S1 1.5V AA CELL PARTS LIST D1 R2 R1 330k EXT BATT IC1 UM3561A LED1 100uF K 1 1k Q1 Q2 SPEAKER S2 Fig.2: here’s how to install the parts on the PC board. If you are going to power the circuit from a 1.5V battery, leave out D1 & LED1 & install links in place of R1 & R2. Alternatively, if you intend powering the circuit from an external supply, leave the battery holder out instead & choose R1 & R2 from Table 1. current being switched through the speaker from causing interference with the operation of the IC. Three components, diode D1, resistor R1 and the green LED, are provided for use only with supply voltages of 3V or more. They can be omitted if the circuit is to be powered from a 1.5V battery. Diode D1 protects the circuit in the event that the supply voltage is accidentally reversed. Resistor R1 and the green LED form a 2.3V regulator to supply the IC. Table 1 shows the suggested values for R1 and R2 for exter­ nal supply voltages of 3V, 6V, 9V and 12V. Note that for the higher supply voltages you need to use a bigger speaker otherwise the power rating of the 50mm speaker will be exceeded. A bigger speaker sounds better too. Finally, we should mention the modification necessary if you want to obtain the machine gun sound. This requires pin 1 to be connected to the Vdd rail (ie, pin 5). In this condition, the setting of the slide switch does not matter and the machine gun sound will be emitted in bursts. Our feeling is that most people will not be interested in the machine gun sound and will build the project only for the siren sounds. Assembling the board Putting the board together won’t take long at all since there are so few parts. If you are going to power the circuit from a 1.5V battery, you will need to install the battery holder on the board and you can leave out D1 and LED1 and install links in place of R1 & R2. If you are going to power the board from 3V or higher, you will need R1, D1 and LED1 and you should leave the battery holder off the board. Fig.2 shows all these components on the board just to show their positions. Make sure you install the transistors, the IC, the diode and the LED the 1 PC board, 77 x 33mm (DSE Cat. ZA 1325) 1 AA single cell holder 1 AA 1.5V alkaline battery 1 UM3561A IC (IC1) 1 BC549 NPN transistor (Q1) 1 BC337 NPN transistor (Q2) 1 green LED (LED1) 1 1N4007 diode (D1) 1 50mm 8Ω loudspeaker 1 SPDT miniature slide switch (S1) 1 3PDT miniature slide switch (S2) 1 100µF 25VW electrolytic capacitor 1 1kΩ 0.25W resistor 1 330kΩ 0.25W resistor Note: see Table 1 for values of R1 & R2 if an external DC supply is to be used. Where to buy the kit This project was designed by Bernie Gilchrist of Dick Smith Electronics who own the copyright on the PC board. Complete kits will be available from all Dick Smith Electronics stores at $9.95. The catalog number is K-5514. correct way around and the same comment applies to the 100µF capacitor. After the components are soldered in, carefully inspect the track side of the PC board for bad joints and solder splashes. You can then connect the battery or your external DC power supply and make siren sounds to your SC heart’s content. November 1993  65