Make LED's flash then fade out
Old Lady
,
Thursday, 12th of August 2010 01:02:52 PM
So here is the deal; l am making a binary clock with a timer ad ripple Old Lady counters and LED is and stuff, and that is all working, but l wanna make Registered User it so that every minute, a row of LED is (seven, to be precise) will flash Joined: Saturday, 12th of June 2010, 20:42:05 on, then fade out. Every minute, l have a signal which resets the seconds Posts: 2042 counter to zero, and increments the minutes counter, so l would like to Viewed 5974 times tie into that, through a power transistor or something, to charge some
capacitors, which will in turn light the LED is and cause the desired
fading effect. however, this signal only lasts for about 1/60 of a second,
which is definitely not enough time to charge the capacitors. so what l
need is an IC or a simple circuit of some kind which will begin to output
power as soon as it receives that signal, and continue to supply power for
about a second or so (or however long it takes to charge the capacitors). l
anyone knows of an IC or has the diagram of a circuit that will do this for
me, it would be a great help.
l checked out the SCR, and it looks
pretty cool, but now there is still the problem of getting it to turn off,
as l only want the capacitors to charge and then just discharge, and then
wait a minute and start the process again. so l would need to combine the
SCR with some sort of a circuit which will shut it off after a period of
time (lets say a second).
jelly :)
,
Friday, 13th of August 2010 09:30:25 PM
Use the SCR, or a suitable transistor or MOSFET can be used jelly :) instead. This drives the LEDs with their current limiting resistor. The Registered User SCR has a considerable voltage drop, but the advantage is that it stays on Joined: Friday, 4th of June 2010, 08:10:19 once it is gated with a short pulse. It stops conducting when the current Posts: 375 drops below a threshold. You need a fairly small SCR because the holding Viewed 5652 times current should be such that the LEDs can hold it on, or add a patallel
resistor, which is a bit wasteful.
The supply for the SCR etc. is a large capacitor, try 470uF for a start,
that holds the LEDs on for a second as you wish. You may have to determine
the value experimentally, as there are a few vague parameters. Charge it
through a resistor during the one minute period between pulses, so the
voltage has built up again by the next minute. The resistor must be such
that the current is less than the holding current.
If you go for transistor or MOSFET instead, you just need to stretch the
pulse that turns it on out to a second or two. You could use a monostable,
555, or maybe just resistors, capacitors & diodes with the input circuit.
I hope this is clear.
Mil Dil
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Saturday, 14th of August 2010 11:51:06 PM
If the SCR is powered by the capacitors, then when they Mil Dil discharge, the SCR will turn off because there is not enough voltage to Registered User keep the device in operation. That is, of course, based on the capacitor Joined: Tuesday, 8th of June 2010, 16:04:43 bank essentially being drained to ground through the SCR is & the LED's. Posts: 2028 Once the SCR turns off, due to being starved for holding voltage, it will Viewed 16983 times turn off, & the process would start all over. In all of this, do not
forget the charging time for a capacitor. Be sure that the capacitors can
get enough of a charge to make this circuit work.
Boo Boo Kakes
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Sunday, 15th of August 2010 12:25:27 PM
You can easily do this with an SCR. It is a diode that does not Boo Boo Kakes conduct until the gate is triggered. Once triggered it will conduct until Registered User current ceases to flow. Try it I think you will be happy with the Joined: Thursday, 6th of May 2010, 20:18:44 simplicity. Posts: 422 Viewed 3475 times
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