Hello Circuit Builders,

I'm a scale model builder and have just begun to add LED lighting to my models.
I am constructing a vehicle with a set of external spotlights using white LEDs. The viewer is meant to be able to press and hold a spring loaded switch which will illuminate the LED spotlights over a span of about 1 second, as if the lights were incandescent bulbs.

When the switch is released, the spotlights should extinguish over roughly the same 1 second time span.

I've found a few examples of similar type circuits here on the forums, but nothing to exactly match what I need.

The model needs to be lightweight and portable and thus will be powered via 11.1 volt LiPo battery hidden in the base. The LEDs I'm planning to use operate at 2.5 volts and draw about 20ma.

I'm still very new to the world of "home brew" electronics so I may need to request that any help be followed up with greater clarification.

Thanks in advance,

John

 

Although you could do this using discrete components a more compact way would be to use a microcontroller. What is the maximum space you have for the electronics?

 

Here is an example of an LED imitating an incandescent lamp's ramp-up and ramp-down luminance profile. It uses a 1.5 volt battery, but you can adapt the circuit to other voltages

The circuit:
(Sorry, cannot get it to display the circuit directly in this thread.
http://cappels.org/dproj/1.5V_Red_fader_blinker/fading_red_led_blinker.gif

The details:
http://cappels.org/dproj/1.5V_Red_fader_blinker/1.5_volt_1970's_style_caboose_marker_flashing_led_light.html

 

How many LEDs? Might do it with 2 resistors, a cap. & darlington transistor + resistors for LEDs. My lights seem to take less than 1/10 sec to come on & 1/2 sec. to turn off.

 

A straight RC charge curve appears visibly as instant on with LED. Reversing the curve would be nice but comprimised on linear charge for turn on. RC discharge used for turn off. LM 334 used for constant current to charge 5 uF cap; R1 adjusts current giving delay on from minutes to rat now. Darlington transistor,U2, presents verry little loading on C1. Times are almost fully adjustable. Continuous load on power supply about 2 uA.