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555 timer

John Sherrill

Jul 20, 2017
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Hello:

I need a 555 timer circuit with 12-volt input and output. When the circuit is turned on, I need a 4 to 5 second output and then dormancy until the circuit is turned off and then on again.

If someone can show me a suitable circuit, I will be very grateful.

John Sherrill
 

Audioguru

Sep 24, 2016
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A 555 timer does not have an input. But it does have a power supply which can be 12V and it has a trigger input that will begin the timing. Its output with no load will go only to 1.3V less than its supply voltage and the voltage drop changes with temperature and with loading. The 555 draws a quiescent current of about 7mA when it is "dormant".

A Cmos 555 is an LMC555, TLC555 or ICM7555. They can have a supply of 12V and with no load their output goes to 12V. Their quiescent current is almost nothing.

The datasheets show the circuits.
 

Harald Kapp

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The circuit you're looking for is a monostable flipflop. It is shown in the datasheet or on many webpages, e.g. here.

The issue with the standard circuit ist that you need to trigger the 555 by a negative edge on the trigger input (pin 2).
This circuit solves the issue with a resistor and capcitor on pin 2. It also has an extra output transistor to increase the available output current. The resistors and capacitors for your timing can be calculated from any web based 555 calculator. Note that at 4-5 seconds it won't be very precise.
What's missing in the above schematic is a 10 nf or 100 nF capacitor from pin 5 tp ground to stabilize the IC.
 

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The circuit shown is missing an important resistor in series with the base of the transistor to limit the base current. Without this resistor then the transistor and/or 555 will be destroyed.
 

Harald Kapp

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Well observed! e.g. 1 kΩ should be suitable for many cases.
 
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