enter image description here

The circuit diagram is above R1,4,6,7:1K R2,5:10K R3:100 ohms C1: 0.047 uF C2,3 :100uF C4:69uF C5:0.1uF

And we finished building the circuit and connected the battery onto it. But the LEDs stayed on instead of blinking. On top of this, we tried to measure frequency with a connection wire connected to the output pin (3) and multimeter but the reading was 0.00 Hz. Also, trying a lower value of resistors didn't help. Is there anything we could possibly be doing wrong? What's causing the LEDs to stay on? Below is the image of the circuit. enter image description here


closed as unclear what you're asking by brhans, Enric Blanco, Dmitry Grigoryev, Dave Tweed May 16 '17 at 21:58

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    \$\begingroup\$ The circuit shown is for a siren but if its not working I suggest you very carefully check your connections as this is liable to be a building error rather than the circuit diagram. \$\endgroup\$ – JIm Dearden May 13 '17 at 10:06
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    \$\begingroup\$ What a rat's nest. C1 appears to be installed backwards. The top 555 (bottom in your schematic) should oscillate too fast to see on the LED. Check pin 3 of that 555. \$\endgroup\$ – Spehro Pefhany May 13 '17 at 10:44
  • \$\begingroup\$ When building on a breadboard, try and keep your component leads as short as possible. Use a cutter to tweak them. Fold them at 90 degree angles. A 250mW resistor (the type you used) should be 4 "squares' long. the capacitors you showed should be similar. They should not be floating like that, but the body of the resistor should be in contact with the breadboard. This will allow you to have much neater breadboarded circuits, and greatly simplify troubleshooting. \$\endgroup\$ – Joren Vaes May 13 '17 at 14:01
  • \$\begingroup\$ Since there is an output voltage which turns on the LEDs, wouldn't there also be a frequency of that? \$\endgroup\$ – Jayden May 13 '17 at 15:09
  • \$\begingroup\$ If the 555 is not oscillating, the LED will be driven by a DC voltage, frequency = 0. \$\endgroup\$ – Peter Bennett May 13 '17 at 15:17

If you read the (translated) site where you got your circuit:

When both timers are in an oscillatory mode, but the frequency of the oscillation circuit IC1 is much smaller than the IC2, IC1 output (pin 3) can be used to modulate the sound generated by the second chip IC2.

You are taking a circuit, which is meant to produce an increasing frequency sound (probably a chirp) using two oscillators (one low and one high) to make two LEDs blink. The frequency of IC2 is too fast to see blink.

What are you trying to accomplish?


Double, even triple check your wiring.

Sometimes large value electrolytic capacitors used as the timing cap (like C4 in your circuit) can have a high ESR value, and won't charge up to your expected value. I've had this happen before with a crappy 1uF cap. Changed brands, and then it worked. If you have a way to measure the voltage across C4, it should be charging up to around 6 volts.

You don't say where you're measuring the frequency. If you're measuring pin 3 of IC1, it's only about 1 Hz, so not sure if your meter will read that. Pin 3 of IC2 will change from the base frequency of about 1500 Hz to the modulated value from IC1, every half second. However, if you're having trouble with C4 not charging up, there will be no frequency to measure, it'll just be on constantly.

Also, if you're using an LED instead of a speaker from IC2, you'll never see it change at those frequencies. It will appear to be on constantly.

You can use http://www.ohmslawcalculator.com/555-astable-calculator to calculate different values for this circuit to better suit your needs.

Finally, (I hope this is allowed here) you can check out my book about the 555 timer to learn more about how it functions and build some example circuits on a breadboard. http://www.tiny.cc/555book


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