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I want to be able to have a set of led's with 2 voltage inputs for a brakelight on a car but i want it to show the nightlight aswell is this possible guys
Hi,
You need to PWM the LED, ie have the LED running at 2 duty cycles. What PWM means is that, you have a fixed frequency, thus fixed period. Let's for example take period to be 10 mili seconds. Then to have 100% brightness, the LED must stay on for 10 mili seconds. For 20% brightness, on for 2 mili seconds and off for 8 mili seconds, ie, have a duty cycle of 20%.
You can use a 555 for PWM. There are many ways you can achieve PWM for LED.
so for instance if i have 30 7000mcd red led,s coming on when lights are turned on with this in line it will allow the same led's to glow brighter when the brake pedal is pushed
Hi,
When you set the new duty cycle, the LEDs all grow at the same brightness, if brighter than all glow brighter, else, all glow dimmer.
It depends on what happens when you press the brake pedal. You could wire it up with a fixed resistor that changes the duty cycle in a 555 astable circuit.
Hope this helps.
Tahmid.
---------- Post added at 21:31 ---------- Previous post was at 21:30 ----------
Hi
I totally agree with betwixt - 2 resistors will do the same job instead of being involved with PWM. In addition there was no request for expanding the solution, so the simplest solution (betwixt - resistors) should be considered as the right one solution (Occam's razor principle).
In theory, two resistors can be used. In practice, to achieve acceptable efficiency, car LED lights would use series connected LEDs and constant current driver, preferably switched mode. If it's too complicated, you better go for traditional incandescent lamps.
In Western Europe, DIY car lights won't get an official allowance, by the way.
I would imagine in an automotive system there would be two switched sources of power, one from the dashboard switch and one from the brake pedal switch. It wouldn't seem efficient to connect them to a PWM generator when resistors would suffice. In a real system there would be a cluster of LEDs but they could be wired in two groups, one for the rear light and one for the brake light, each group consisting of several series LEDs and resistors to make up the required brightness. If all the LED chains were wired with a resistor then paralleled so there were only two external connections, it would still work at two brightness levels, brake with power applied directly to the LED cluster and rear light powered through a single fixed resistor. Depending on other factors in the vehicle, it might be necessary to fit blocking diodes to isolate the power feeds.
Consider the difference in cost between a few resistors and the components needed for a PWM generator and the need for additional wiring to power the PWM circuits.
If the LED brightness had to be variable, I would agree PWM is the method to use but for just two levels it would be overkill.
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