Using a transistor to dim a dc bulb

[kybert]

Hi,

Can someone help me construct a circuit to dim a filiment lamp (9Vdc) using a transistor?

The bulb draws 38mA when connected to 9Vdc, so its a 0.34W bulb.


I have attached my circuit. The lamp can be shorted out by a push switch (this is a door bell push switch with intergrated lamp), so R22 (50 ohms) limits the current in the circuit to 160mA during a button press.

How do i calculate the value for the variable resistor? Is this curcuit correct?

When the button is pressed, Q6 should turn off, hence the micro detects the button press?

 

[crutschow]

The circuit will not work well since it operates by starving the base current and depends upon the gain of Q5 to determine the operating point. This makes it rather temperature sensitive.

A better way is to remove pot R23 point E from Q5's base and connect point E to ground. Then remove the wiper of the pot from point A and connect to Q5's base. For current limit at the pot extremes, add a 10khm: resistor in series with the pot wiper and Q5's base. (You don't show the value of R23 but good values are 5Khm: to 50Khm:.)

Your notation for Q6's collector output is reversed. When the button is pressed Q6's collector output will be high.

 

[Thunderchild]

pray why MUST the lamp be shorted when the button is pressed ?

 

[kybert]

pray why MUST the lamp be shorted when the button is pressed ?

Becuase its a door bell press button, and the lamp is built into the button.

Ive decided not to dim the lamp, just limit the current with a 51R 2W resistor, so much easier!

 

[Thunderchild]

ok i gotcha yes thats the best Idea, usually the bel provides so current for the light and when the switch us pressed it shorts the bulb and gives full power to the bell so that it actuallt rings

 

[shokjok]

A MOSFET as an oscillator, with a CdS photocell in the circuit, can be used to automatically dim the LED, if you insist upon using a transistor. Is the LED in the doorbell switch cover, or a part of the switch?

 

[mneary]

Where did you find an LED? The OP is using a common doorbell switch with a 9V, 38mA filament (incandescent) lamp.