need help with transistor switching circuit

[ant9985]

hi, can anyone tell me how i would design a transistor switching circuit to light an LED as a warning, when the ambient light falls below a particular value. any help appreciated

 

[zachtheterrible]

i think ive got exactly wut ur lookin 4.

ur supply voltage can be anything that the transistor can handle.

the variable resistor there can be a pot, or a fixed value. depends on how sensitive to light u want it. the lower the value, the more sensitive, vica versa.

just make sure to use a resistor to limit current @ the emitter.
i think if u use a 1.5v or 1v, u probly shouldnt need the resistor.

 

[ant9985]

oh right, thnx a lot

 

[Exo]

There should be a resistor in series with the pot.
If you turn the pot all the way up there will be no base resistance for the transistor wich will cause it to die horrobly

 

[zachtheterrible]

oh yea, frgot to mention that, hope ya havent built it alredy :lol:

 

[ant9985]

naa, not yet thnx for the tip exo

 

[ant9985]

i have built the device on multi-sim (simulation program). i set the top resistor on the diagram as 51ohms, the one in series with that as 20 ohm and the bottom one on the diagram as a potentiometer of 1k. and the transistor i used was 2n3860. i also connected a vcc power supply of 5 volts at the top. the LED is allways lit. is the circuit working correctly? do i need to do any tests to see? just out of interest, where does the light enter?

 

[Nigel Goodwin]

This circuit is particularly crude (which is why it's so simple), and it won't perform well. Have a look at figure 12 for a far better design. You could improve it further by adding a resistor from pin 6 to pin 3, giving hysteresis to the switching action - a high value, 1M or higher?, is all that's required.

 

[ant9985]

oh right, thnx nigel. the circuit seems to be working fine now, its only for a college assignment i dont actually have to build it