Various types of siren circuit like electronic siren, toy siren, warning siren etc. are already posted in bestengineeringrpojects.com . Now here in one another simple siren circuit using popular and low-cost NAND gate IC (CD4700), which gives hour of entertainment and also can be used in toy.
Description of Simple Siren Circuit using CD7400
Siren circuit is build around NAND gate IC (CD7400), a PNP transistor (SK100B) and few passive components. In this circuit we use only half part of IC1. NAND gates N1 and N2 are wired as low frequency oscillator to generate low frequency and NAND gates N3 and N4 as high frequency oscillator to generate high frequency.
Diode D1 connect when output of low frequency becomes low which short circuit resistors R3 and R4. When output of high frequency oscillator is high, diode D1 does not connect and resistors R3 with R4 are included in the circuit of high frequency oscillator. The high frequency output changes periodically by resistors R3 and R4.
The output from NAND gate (pin 8) is connected to base of PNP transistor T1. Transistor T1 is used as an amplifier and drive an 8Ω loudspeaker connected in collector. This circuit works well with ordinary 3 torch-cell batteries.
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PARTS LIST OF SIMPLE SIREN CIRCUIT USING CD7400
| Resistor (all ¼-watt, ± 5% Carbon) |
| R1, R2, R6 = 2.2 KΩ
R3 = 10 KΩ R4 = 1.2 KΩ R5 = 2.7 KΩ |
| Capacitors |
| C1, C2 = 25 µF, 25V (Electrolytic Capacitor)
C3, C4 = 0.05 µF (Ceramic Disc) |
| Semiconductors |
| IC1 = CD7400 (CMOS Quad two input NAND Gate IC)
T1 = SK100B (Medium Power silicon PNP Transistor) D1 = 1N4148 ( Small Signal Diode) |
| Miscellaneous |
| LS1 = 8 Ω Loud Speaker |