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help with making an led counter (a bit like a vu meter) but different? Answered

i am trying to make a night light that whena button is pressed turns ON lights up a row of 10 leds. everytime the circuit gets a pulse from the 555 1 led goes OFF till all 10 are off then the lights stay off till i press a button again i know that i will need to use a 555 chip to make the pulse to another ic but im not sure about which other ic to use and how to wire it up thanks :) Andy


there is a circuit that uses 2 or 3 555 timers to do this. You would have to look it up because I saw it in a magazine and I have no idea if its on the internet.

It's actually quite difficult to do this simply - but I'm thinking about it. The simplest way is to use a small 10 bit wide ROM driven from a binary counter. Alternatively, it's easy to get close with each LED lighting up in sequence rather than leaving the previous ones on using a 7442 BCD to decimal decoder driven from a decade counter. Does this have to count clock cycles? There is a device (LM3914) which will light up LEDs in turn with an increasing analogue voltage on the input. I've got it . . you use a shift register. Tie the input high, pulse the reset and you've got all zeros. You'll get the bar building up with successive clocks and could make it as long as you like. You'd have to buffer the outputs to drive the LEDs though. Are you familiar with digital logic, or is this all gibberish to you. Give me details of the application and I may be able to help you a bit more.

thanks for the reply. im only very new to electronics so you are right that most of that was gibberish :-) i have just read up about the lm3914 and 15 and i think i can use that for a diferent project i am doing. the application that i am trying to make is a night light for my room, i have changed the specification in the question because i had wrote it wrong

That gives me a better idea. You're looking for a controllable brightness night-light, with more LEDs coming on as you press a button. This is the basic LM3914 circuit you will need to drive a column of LEDs. This would be easy to control with a potentiometer. If you want to control it with a button you'll need a bit more circuitry.
(I'm away for a day or 2 - I'll pick this up when I get back if no-one's already done it.)

I see - From the revised spec you want a night light which will slowly fade out over a period until you turn it on again. Shift registers are the way to go and this is actually easier then before. I'll give it some thought over the weekend. Of course, if you want to get into microcontroller programming as part of your hobby this would be an absolutely perfect project to get you started with Arduino, ATTiny or PicAxe.

My understanding is you want to rather 'physically' dim the light slowly after having it turn on... If thats the case, then making an analog dimming led would be quick and easy - a double pole double throw switch, 2 'large' capacitors, led, matching resistor, and power supply... The dpdt switch connects both commons to either the A side or the B side. When the switch is in position A, it charges capacitor B, and connects the led to capacitor A. When the switch is in the B position, it switches - using B for lighting the led and charging A. For the countdown design you want - replace the 1 led with the lm3914 circuit - it will show the number of leds based on the charge in the capacitors, which will slowly drain powering the lm chip.


If you have an Arduino, or something similar (Freeduino, Boarduino, etc) you could do this quite easily.

A 4 stage binary counter would do the trick I think: You could hook up the 1st, 2nd and 4th pin to AND ports and connect this to the reset of the binary counter so it starts counting all over again.