Optimize 555 Timer Chip Output | Transistor Configuration

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Discussion Overview

The discussion focuses on optimizing the output current from a 555 timer chip using various transistor configurations. Participants explore potential solutions to achieve higher output currents, specifically aiming for 200 mA, while addressing issues related to voltage supply and circuit design.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant seeks advice on achieving higher output current from a 555 timer chip, currently limited to 30 mA.
  • Another participant suggests that with the right transistor and voltage, achieving 200 mA should be possible and offers to help if a schematic is provided.
  • Discussion includes the potential effectiveness of both common emitter and common collector configurations, noting that common emitter inverts the voltage while common collector does not.
  • A participant questions the resistor value between the transistor emitter and the LEDs, suggesting that a 5 V supply might be limiting the current output.
  • Concerns are raised about whether switching to a 9V supply would affect the frequency of the oscillators and if it would help increase current output.
  • Another participant notes that the transistor may not be operating in a constant current mode due to voltage drops and suggests that the beta of the transistor might be low.
  • One participant expresses uncertainty about changing the supply voltage and its effect on the output frequencies of the timers.
  • Another participant indicates that changing the voltage would likely only cause a slight frequency change, which may not significantly affect detection.
  • Discussion includes the measurement of current and duty cycle, with one participant explaining how the duty cycle could affect perceived current readings.
  • A later reply mentions achieving 200 mA with a specific transistor configuration, although the details of this success are not elaborated upon.

Areas of Agreement / Disagreement

Participants express differing views on the effects of supply voltage on current output and frequency, as well as the operational mode of the transistor. There is no consensus on the best approach to achieve the desired output current, and multiple competing views remain.

Contextual Notes

Limitations include the lack of specific resistor values and the uncertainty regarding the operational characteristics of the transistor under different voltage conditions. The discussion also reflects varying assumptions about the circuit's design and component interactions.

jbord39
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What would be the best configuration for amplifying output from 555 timer chip? No matter what transistor configuration I cannot get it higher than 30 mA (I am shooting for 200 mA).

Thanks
 
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You should be able to get more than 30 mA, even 200 mA depending on the transistor, voltage and load. If you post your schematic, maybe we can help you.

Both common emitter and common collector may work. Common emitter will invert the voltage from the 555 timer. Common collector will result in a maximum output voltage slightly less than the output voltage of the 555.
 
Thanks. Here is my schematic; I cannot remember resistor values, but the 556 output is 350 Hz and 38000 Hz.

John

remoteupdated.png
 
What value is the resistor between the transistor emitter and the LEDs? I think the problem is the use of a 5 V supply. Is there any reason you can't use a higher voltage?
 
Would switching to maybe 9V change the frequency of both oscillators?

The value of the resistor b/w the TIP120 and the LED's (Which have Vf = 1.2V) is .5 ohms. I gradually decreased it from like 50 ohms trying to get more current but I cannot. (Minimum ~ 15 mA, maximum ~ 30 mA).

Thanks,

John

Edit: Maybe the transistor's base is at a lower voltage (due to voltage lost inside the 556?) then the +5V collector voltage. Could this cause the TIP120 to operate in the triode region rather than constant current? If so, would applying 9V to the collector instead of 5V increase the current? I am getting power from a 9V battery which is regulated to 5V with the LM317; so it would be easy to connect the collector to +9V instead of +5V.
 
jbord39 said:
Would switching to maybe 9V change the frequency of both oscillators?
Maybe the transistor's base is at a lower voltage (due to voltage lost inside the 556?) then the +5V collector voltage.
Yes and not only that, you're going to lose another ~0.65V through the transistor.

jbord39 said:
Could this cause the TIP120 to operate in the triode region rather than constant current? If so, would applying 9V to the collector instead of 5V increase the current? I am getting power from a 9V battery which is regulated to 5V with the LM317; so it would be easy to connect the collector to +9V instead of +5V.
I wouldn't say the transistor is operating in either the triode region or constant current mode. In this case the current is determined by the amount of current supplied by the 556 and the beta of the transistor, provided there is enough voltage available. It is also possible that the beta of the transistor is low and the 556 can't supply enough current. I don't think just connecting the collector to 9V will help unless you also connect the 556 to 9V too.
 
Thanks for the reply.

My problem is that the circuit is already soldered, so I was wondering how much you think changing the supply voltage from regulated +5 to 9V battery would change the output frequencies of the timers.

Thanks,

John
 
Changing the voltage will change the frequency very slightly, probably much less than the tolerance of your components. I'd be surprised if it shifted the frequency so much that your detector stopped detecting it.
 
It depends on how you measure it, too.

If you are just using a meter to measure it, you will get a reading of average current.
But in this case the duty cycle will be about 25% or so.

You have a square wave turning on and off at 38 KHz, so that probably has a duty cycle of about 50%, and then you are turning this square wave on and off with a 350 Hz square wave. So, the duty cycle could be something like 25%, or maybe even less.

So, to get a current reading of 30 mA, the actual current might be more like 120 mA when it is flowing through the LEDs.
 
  • #10
I am pretty sure my DMM is made to measure square wave currents; but to make sure I also monitored the voltage across the resistor with an oscilloscope, and it was only getting 30 mA. I ended up getting 200 mA using the transistor configuration though.

Thanks guys,

John
 

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