PhotoTransistor and Hex Schmitt trigger Inverter

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SUMMARY

The discussion centers on troubleshooting a circuit involving a phototransistor and a Hex Schmitt trigger inverter. The user, Anita, initially reported a low output voltage of ~240mV, which was insufficient for the Schmitt trigger to function properly. Recommendations included increasing the resistance of R2 and adjusting the supply voltage (Vcc) to 5 volts for optimal performance. The ideal Vcc for the Schmitt trigger is noted as 3.3V, but a higher voltage may enhance the trigger's sensitivity.

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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
 

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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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