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Question on a simple project
Posted: Wed Jun 25, 2003 7:51 pm
I am building a launch device with countdown for model rockets. The main focus is a 10 led bar from radioshack. Using a 555 and 4017 to run them. I have them ticking from left to right but what I want is to have them all on when #1 is lit, all but #1 when #2 is lit.. and so on so it seems as a bar moving to the right. Ripple blanking MSB sort of. I thought I could accomplish this with diodes on the cathode side of the leds, but I seem to get nothing... Why aren't the diodes working? I am using +9V DC<p>Any and all help is appreciated.<p>[ June 25, 2003: Message edited by: PainGypsy ]</p>
Re: Question on a simple project
Posted: Thu Jun 26, 2003 6:54 am
The most likely culprit to your problem is the voltage drop across the extra diodes you're adding. Red LEDs need roughly 1.7 V across them to light, and diodes like the 1N914 or 1N4001 need 0.7 V before they begin conducting in any sizable quantity. <p>When you use a resistor in series with the LED, you are doing this because the resistor takes the remaining (9V - 1.7V) at a current of roughly 10 to 20mA for the LED. To calculate such a resistor value, take (9V - 1.7V) / 10 mA = (9V - 1.7V) / .01A = 730 Ohms. Due to Kirchoff's current laws, no more current can flow through the LED than through the series resistor, and Ohms law dictates what the current through the resistor is.<p>Anyway, when you get a diode in series with the LED, especially when current is flowing through many diodes (which I understand are connected between each cathode of the LEDs), too much voltage drop is taking place. With just one or two diodes to go through, the current may drop a little, but probably not too much to decrease visibility. But with all 10 LEDs lit, that's (9V - 1.7V - 9*0.7V) = 1V, which would only let about 1.3mA through, which probably isn't enough to light the LED at the end.<p>You may want to look into using the LM3914, though that takes a variable voltage range (between 0 and 9V in your power supply situation) to drive a bar LED in dot or the bar mode you want. See http://www.uoguelph.ca/~antoon/gadgets/lm3914.htm
or google with lm3914. I'm not sure how your circuit could come up with the variable voltage.<p>Your best bet is to use CMOS logic gates (the CD4000 series of ICs) that are arranged to compare each output of the 4017 to the neighboring segments' status. You should be able to do this with some combination of inverters, NOR, NAND, and possibly XOR gates.