Blinking LED at two different frequencies

Question

I am trying to build a circuit that will make a LED to blink at a certain frequency, probably by using 555 timer. As the circuit will be supplied by a 3 Volt battery, so I want to extend a feature that LED should start blinking at a higher frequency indicating low battery status but I dont know how to add this feature to the circuit without adding any other IC or comparator.

Any help is greatly appreciated.

I know it would be a easier and useful way, but I'd like to know If there is any other option other than it. — Arun Lal, Sep 07 '16 at 09:50
A relaxation oscillator would do the opposite, giving you a higher frequency with higher voltage. But this could be reversed by working out a way to divert away more current from a capacitor, with higher voltages present. Do you want an analog solution or a micro? What's your "low bat" voltage status? How much load can the circuit present? — jonk, Sep 07 '16 at 09:51
If you're trying to build a circuit, why don't you share the results? — Dmitry Grigoryev, Sep 07 '16 at 10:20
@ArunLal Please be specific and ask a question. Also provide supporting materials (like a schematic, use the circuit editor) for your solution and where you are experiencing a problem in your design. — Voltage Spike, Sep 07 '16 at 16:05

score 1 · Answer 1 · edited Jun 11 '20 at 15:10

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Firstly I would advise using the CMOS version which operates at a lower voltage than the BJT version 555.

The key to getting different flash rates is using the control pin. This is connected to the internal resistor ladder (3 x 5k or 5 5 5).

Adding a jelly bean NPN and two resistors you can easily switch between the two rates. When the transistor is turned ON the flash rate goes up. With the transistor OFF it goes down. Now all that is required to complete your project is some form of comparator switch to operate the transistor e.g. a low voltage rail-to-rail op amp.

edited Jun 11 '20 at 15:10

Community

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answered Sep 07 '16 at 11:29

JIm Dearden

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Change the NPN to connect the emitter to ground. – WhatRoughBeast Sep 07 '16 at 12:36
@WhatRoughBeast You could do that or even make R2 variable - the value isn't that critical- it simply goes in parallel with the internal 5ks to change the upper and lower switch points. – JIm Dearden Sep 07 '16 at 15:37

score 0 · Answer 2 · answered Sep 07 '16 at 11:52

Use two resistors in series instead of one that determines the time period.

Include a switch(or transistor) across any of these resistors.

Frequencies will be different in with and without switch press(or with trigger and without trigger to the transistor)

Blinking LED at two different frequencies

2 Answers2