Nesgodas

Hello, I’m a novice or worse, but can assemble things with the proper drawings. I’m trying to find a schematic that will light up 7 leds in sequence, leaving each of the leds on until the power is turned off, as depicted in this link.
YouTube - LAWGIVER MARK 2
Any help would be greatly appreciated.
Thanks in advance.

HarveyH42

You could probably use an LM3914 to measure the charging of a capacitor. Simple and cheap. Sorry, I don't have schematic for you, might be in the datasheet...

audioguru

An LM3914 is a linear voltmeter that needs the capacitor to charge with a constant current circuit.
An LM3015 is a logarithmic level sensor that shows a linear response to a capacitor's voltage that is charged by just a resistor.

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Uncle $crooge

Mikebits

I am not following you here. Both the 3914 and 3915 have the exact same input circuitry. The only difference between the two parts is the resistor divider chain for the compare circuits.

ericgibbs

Morning Mike,
As agu is most likely sleeping, maybe I can help.

Its the resistor divider that set the difference twixt the linear and log thresholds on the individual comparators.

The LM3914 resistors are the same value, whereas the LM3915 resistors form a logramithic chain, mostly used in audio sound level indication.

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Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

MrDEB

an easier and cheaper way would be a serial in, parallel out circuit using a 74ls175 or the cmos version?
add a 7555 square wave clock and your done.
Might have to alter the outputs w/ transistors for a brighter light.
just a thought.

audioguru

If you use a high speed Cmos shift register then each output can be up to 25mA.

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Uncle $crooge

HarveyH42

Might as well just use a PIC... Think I remember seeing the PICAXE starter kit was only around $15 (Canadian)...

What's the output current of the LM3915? Thought this was the sort of thing it was designed for...

audioguru

The output current from an LM3915 is programmed up to 30mA per output.
But if the supply voltage is higher than 6V and the LEDs are 2V red ones the LM3915 will overheat if all 10 LEDs are lighted. a resistor can be added in series with the group of LEDs to share the heat.

I think the exponential voltage rise of a capacitor charged by a simple resistor matches the logarithmic voltage response of an LM3915. Each LED would light with the same amount of time between steps. An LM3914 is linear and the capacitor must be charged with a constant current source circuit to do the same.

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Uncle $crooge

crutschow

I could be wrong, but I think the logarithmic response is in the wrong direction. The response of the LM3915 is most sensitive at low voltages where the RC charging voltage would be moving most rapidly. Thus the first few LEDS would light very rapidly with a signficant slow down at the end.

If you charged the cap to the high voltage level and monitored the RC discharge, then I think the lights would turn off in a linear fashion. You could get a sequential linear turn-on in this fashion if you added inverters to each LED output.

Or is my thinking backwards?

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Carl

audioguru

You are correct. The charging is wrong.
I graphed the discharge. It is correct.

Attached Images

RC discharge.PNG (8.3 KB, 4 views)

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Uncle $crooge

shokjok

What about a JFET or MOSFET used as an oscillator to drive a 74LS175, or series flip-flops as sequential bucket brigade registers?