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Author Topic: PIC triggering 555s?  (Read 2464 times)

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Colonel Shofer

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PIC triggering 555s?
« on: January 25, 2012, 07:42:03 AM »
Howdie all,

Can we 'trigger' a 555 chip, from a PIC digital output (with other components to the 555, I expect), so that we can control the 555 (which then clocks at its own, set, frequency)? e.g. so that a few 555s could be used to breath LEDs, as the PIC continues through its program?

TYIA.

CS

kam

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Re: PIC triggering 555s?
« Reply #1 on: January 25, 2012, 08:24:23 AM »
a typical PIC can source 20-25mA of current (this is not standard - you need to check the specifications first) and the 555 operates from 3 to 15 mA (depends on the setup). So theoretically you can source a 555 directly from the PIC output. If you want also to source the 555's output load, then you need to add this current as well. Better use a transistor driver to control the 555, just to make sure that nothing will go wrong.

I made a keycode lock a year ago, and a used 2 PICs, the first was the primary and the second the secondary. The secondary PIC was sourced completely from a pin from the primary with no driver whatsoever. So yes, it is absolutely possible to do this, as long as you do not exceed the pin DC source current

Colonel Shofer

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Re: PIC triggering 555s?
« Reply #2 on: January 26, 2012, 08:04:07 AM »
TY K,

Super ...

So, looking at your 555 breathing circuits, how would the PIC be connected to the 555 (via the transistor driver)?

TYIA

CS

kam

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Re: PIC triggering 555s?
« Reply #3 on: January 26, 2012, 09:48:50 AM »
what will be the LED supply voltage? 5V is ok?

Colonel Shofer

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Re: PIC triggering 555s?
« Reply #4 on: January 26, 2012, 10:40:52 AM »
As a start, that will be fine TY :)

CS

kam

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Re: PIC triggering 555s?
« Reply #5 on: January 27, 2012, 09:12:14 AM »
do, for normal LEDs (not high brightness) i recommend this circuit http://pcbheaven.com/circuitpages/555_Breathing_Pulsing_LED/

The output of the PIC goes through a limiting resistor to the base of a NPN transistor. I will tell you how to calculate this resistor later. Then connect the collector of the transistor to the 5 volts. Use now the emitter as the power supply for the breathing circuit. This is a typical emitter follower. It will work better if the supply is little above 5 volts.

For the resistor, if you use a darlington pair transistor then you can safely use anything between 1kohm and 2.2KOhms. That is because such a transistor has a huge hfe and you do not have to worry about.
On a normal transistor, hfe is usually 80 to 200. Use worst case scenario (80) to calculate your resistor. You need to know the current that the breather will draw, and that is the breather current plus the LED current. Let's assume that all together draws 200mA (the breather needs some 30mA to operate). So, the Ic current is 200.

Ib = Ic/hfe => Ib = 200/80  = 2.5mA

You need at least 2.5mA base current to operate normally. Do not be cheap and give it some space to breath. Multiply this by 10, which makes 25mA. Now you can calculate your resistor. The supply from the PIC is 5V:

R = V/I => R - 5/25 = 200 Ohms

Ok, a 330 Ohms resistor will do just fine.



_pike

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Re: PIC triggering 555s?
« Reply #6 on: February 13, 2012, 11:52:15 AM »
May i ask? Why don't you use the pic to do the whole job?