Author Topic: 555, Frequency increase under load  (Read 1693 times)

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Offline BlastcapTopic starter

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555, Frequency increase under load
« on: May 18, 2016, 12:23:58 pm »
I was playing around with a 555 timer, and wanted to add a "Buzzer", to decrease the load on the 555 output, i added a 547 transistor and used that to drive a small 4 Ohm speaker.

When the circuit was "idle" (everything hooked up, but left the speaker Ground floating) the timer gave a ca. 60Hz output (as expected), but when i connected the speaker, the frequency went up by a factor of 5 to ca. 300Hz.  :wtf:

Frequencies was measures with a DSO5072P, and by ear.

Is this normal or am i doing something very wrong?

Ill add a picture and diagram as soon as i get home.
 

Online Andy Watson

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Re: 555, Frequency increase under load
« Reply #1 on: May 18, 2016, 12:43:59 pm »
4 ohms across what supply voltage? Lets assume your supply is 5V, I make that 1.25A pulses. I suspect your power supply is sagging, if not completely collapsing! If the pulse alone is not sufficient to reset the 555, the momentary change in Vcc will affect the upper trip-point of the timing cycle.
 

Offline Ammar

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Re: 555, Frequency increase under load
« Reply #2 on: May 19, 2016, 12:34:36 am »
What value resistor do you have between pin 3 (output) off your 555 and the base of your transistor?
 

Offline danadak

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Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 

Offline BlastcapTopic starter

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Re: 555, Frequency increase under load
« Reply #4 on: May 19, 2016, 07:54:17 am »
@Andy Watson
You are most probably right, when i read my post again its rather obvious. :palm:


@Ammar
1K if i remember right, might even be less.

@Dana
Thanks for the hint, ill add one as a good measure.  :-+
 

Offline Audioguru

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Re: 555, Frequency increase under load
« Reply #5 on: May 20, 2016, 02:23:44 am »
A BC547 cannot drive a speaker. Its maximum allowed current is only 100mA which produces only 0.04W for a squarewave into a 4 ohm speaker.
Was the transistor an emitter-follower? Then its base-emitter diode overloaded the 555 output.
 


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