LED Dimmer problem

Allen Bong · 2005-12-21T02:16:02+00:00

Hi Group,My friend designed a circuit to light up an LED when a switch ispressed. The led lighted would be duplicated to another identicalcircuit board via RS232 link (2400 b/s). When an LED is lighted, theLED would flash at 1 Hz interval. One would acknowledge by pressingthe corresponding switch on the board. Once acknowledged, the LEDwould be on permanently until another switch is pressed.The switches and LED are arranged as 2 rows of 12 . Only one switch isallowed per row so there can only be 2 LED lighted and 2 LED inflashing condition.After he completed the circuit and fabricated the PCB, he wanted to adda dimmer so he could control the brightness of all the LEDs (thecircuit is used in a boat). As you can see from the circuit below, heturned the LED on by sourcing the LEDs and all the LEDs were common atground through 220 ohm resistors. So what I did is gathering all thecathods and connected them to the drain of a MOSFET BUZ71A. Thisworked fine when the brightness is high. But when the LEDs weredimmed, the permanent lighted LED danced with the flashing LEDs and Icouldn't find a simple solution to solve it.If instead he had sinked the current to switch on the LEDs, I wouldhave combined all the anodes of the LEDs and connected them to a LM317and supplying 2V to 5V to the LEDs. But that would be major work forhim and also rewriting the PIC software.Can someone help? .-------------------------------------------. | | | PIC | '-------------------------------------------' VCC VCC VCC + + + pulsed dc at 1Hz | | | | |LED |LED |LED V -> V -> V -> V -> -LED - - - VCC | | | |...

Dec 21, 2005 20 Replies

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Bob Monsen 20 years ago

Yes, a 2N4403 is a PNP transistor. That won't work.

The Vce(sat) parameter is a characteristic of the transistor's construction. It basically is the smallest voltage between emitter and collector that you can get while passing current.

Glad it worked out for you.

Regards, Bob Monsen "In the survival of favoured individuals and races, during the constantly-recurring struggle for existence, we see a powerful and ever-acting form of selection." -- Charles Darwin

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