PhotoTransistor and Hex Schmitt trigger Inverter

[anita1984]

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,
Anita

 

[waht]

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.

 

[anita1984]

Hi, thank you for your reply,

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

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

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

 

[dlgoff]

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"

 

[waht]

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

 

[vk6kro]

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.

 

[anita1984]

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

 

[uart]

1. Increase R2 until you do get enough voltage.

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