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Home > Circuits > 555 Siren Generator
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Modified:20:32, 22 October 2013
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| INTRODUCTION |
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We were modifying an old retro Radio-Controlled car and decided it should have a siren attached to the spare channel. I wanted a bit of versatility with the sound so rather than using a standard siren, I decided it would have more educational value to build a siren generator instead..
It is based on two 555 timers connected together with adjustments on frequency and utilising the rarely used pin 5 on the 555 IC.
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| CIRCUIT DESCRIPTION |
CIRCUIT DIAGRAM
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CIRCUIT EXPLANATION
IC1 forms the low frequency astable which warbbles or alternates the sound. The frequency is controlled by VR1 which provides a range from about 2-100Hz.
The output is fed via VR2 to the FM input on the second astable, formed by IC2. Normally the frequency of the astable ranges from about 150Hz-10KHz. When an additional voltage is fed to the FM input, the frequency falls in proportion to the amount of voltage present. By varying VR2 combined with the internal resistance of the input, the amount of frequency change can be adjusted. This will allow various 2-tone siren sounds.
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| By adding a capacitor to the FM input, the frequency will ramp up and down, although this will be an exponential ramp, not linear, but the effect is very similar, if not preferable. The effect can be seen on a simulated trace. |
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| The output is amplified using Q1 which is a Darlington transistor capable of 3A output. The circuit was run from the 7.2v battery on the RC car and was VERY loud. The transistor barely even got warm when driving an 8ohm piezo bullet tweeter. You can use a supply range from 6v to 15v but take care on the load on the transistor.
Be aware that the output to a speaker or sounder is quite brutal and switches the supply directly to the speaker/tweeter. The one we used was out of an old speaker cabinet. A new case was made specifically for the tweeter.
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| PCB LAYOUT & CONNECTION |
The PCB is shown right. IC holders were used to hold the ICs
CN1 is a 2pin header connector that acts as SW1.
Connector TB1 connects to the supply and TB2 to the speaker/tweeter.
Q1 can be bent down flat to the board to keep the PCB profile low.
COMPONENT LIST
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RESISTORS
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| R1-5 |
1 KOhm |
5
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| R6 |
4.7 KOhm |
1
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| VR1,3 |
100 KOhm |
2
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| VR2 |
100 KOhm |
1
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CAPACITORS
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| C1 |
4.7 uF Electrolytic |
1
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| C2 |
100 uF Electrolytic |
1
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| C3 |
47 nF Polyester |
1
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| C4,5 |
10 uF Electrolytic |
2
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SEMI-CONDUCTORS
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| Q1 |
TIP122 |
1
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| IC1,2 |
NE555 |
2
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MISC
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| TB1,2 |
2 way PCB Terminal Block |
2
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PCB size: 60mm (2.7in) x 30mm (1.35in)
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Designed and Written by Phil Townshend 2012
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www.edutek.ltd.uk - Working Electronics For Students & Teachers
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