PhotoTransistor and Hex Schmitt trigger Inverter

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Discussion Overview

The discussion revolves around troubleshooting a circuit involving a phototransistor and a hex Schmitt trigger inverter. Participants are addressing issues related to output voltage and the conditions affecting the circuit's performance, focusing on both theoretical and practical aspects of the components involved.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Anita reports no voltage on Vout and seeks assistance in identifying the mistake in her circuit.
  • One participant suggests checking the voltage on the emitter when the circuit is turned on, indicating that low Vcc on the Schmitt trigger could affect the trigger window.
  • Anita mentions a measured voltage of ~240mV at the emitter and asks for recommendations.
  • Another participant questions the threshold levels for the Schmitt trigger chip, providing potential threshold values based on a quick datasheet check.
  • A participant advises that across R2, the expected voltage should be 0 volts in the absence of light and 3 volts when light is present, indicating that insufficient light may be causing the low voltage reading.
  • Anita acknowledges the issue of insufficient light and reports a new voltage of 2.2 volts across R2, asking if this is adequate for the Schmitt trigger to function properly.
  • One participant suggests increasing R2 to achieve enough voltage and mentions that a Vcc of 5 volts might yield better results.

Areas of Agreement / Disagreement

Participants express varying opinions on the appropriate voltage levels and configurations for the circuit. There is no consensus on the exact solution, as multiple suggestions and conditions are presented without agreement on a definitive approach.

Contextual Notes

Participants reference specific voltage levels and component characteristics, but there are unresolved assumptions regarding the light conditions and the specific Schmitt trigger model being used. The discussion does not clarify the impact of these factors on the circuit's performance.

anita1984
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Hello forum , i would like to ask you about this circuit in the attachment , i don't have any voltage on Vout , could you tell me where is my mistake.
Thank you in advance,:cool:
Anita
 

Attachments

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What voltages are you getting on the emitter, when turned on?

Low Vcc on the Schmitt trigger minimizes the trigger window. You could be in a constant on or off state, and because of the sensitivity of the photodetector, it's unable to bring the transistor to a full low state or high state depending on your configuration.
 
Hi, thank you for your reply,
what said:
What voltages are you getting on the emitter, when turned on?
the voltage is ~240mV,what do you recommand me ?

what said:
Low Vcc on the Schmitt trigger minimizes the trigger window.
Vcc=3.3V it's the ideal voltage for the schmitt trigger referring to the datasheet
Have a nice day,
Anita
 
The trigger is so named because the output retains its value until the input changes sufficiently to trigger a change.
http://en.wikipedia.org/wiki/Schmitt_trigger"
 
Last edited by a moderator:
anita1984 said:
Hi, thank you for your reply,

the voltage is ~240mV,what do you recommand me ?

What are the threshold levels for this chip?

I checked a datasheet real quick, may or may not be what you have:

Vcc = 4.5v
V+ = 2.38v
V- = 1.4v
 
Across R2, you should get 0 Volts when there is no light and 3 volts when there is light.

R2 is 1200 ohms and the current through it should be 2.5mA.
So, V = 1200 ohms * 0.0025 Amps = 3 volts.

If you are only getting 240 mV across R2 when it is lit (and presumably 0 volts when it is dark) then you don't have enough light getting to the phototransistor.

This would be why your Schmitt trigger isn't giving any output.
 
Hello , thanks for your reply , it is true that not enought light , i change the system and now i have across R2 2.2 Volt , is it enought for the schmitt to work properelly?
Thank you in advance ,
Anita
 
1. Increase R2 until you do get enough voltage.

2. You might be a bit better off with Vcc=5 volts
 

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