You are right, 12 V can fry a LED, even when set up to measure LED Vf. If we should mistakenly connect pot
to O ohms to start rather than max R, then Poof, LED & or mA meter. That is why I almost always add some resistance in series with a pot, smallest R that will prevent damage by itself.
Simple setup to measure LED Vf, generally used to sort Vf of many LEDs into piles of same V, then if used in series strings, the strings can be made up of low V, med. V & high V so that all strings are = V. The blue LED is used for protection of reversed DUT, device under test. When testing white LEDs, for blue LED might use a white & red LED in series to give about 5 V, which should be safe for a reversed LED. The 470 ohm R could be made from a combination
of series & parallel Rs to give desired test current.

View attachment 108474
View attachment 108474

Tried this and the results are:

12.41v supply
463ohms resistor
1.87 vf

You are right, 12 V can fry a LED, even when set up to measure LED Vf. If we should mistakenly connect pot
to O ohms to start rather than max R, then Poof, LED & or mA meter. That is why I almost always add some resistance in series with a pot, smallest R that will prevent damage by itself.
Simple setup to measure LED Vf, generally used to sort Vf of many LEDs into piles of same V, then if used in series strings, the strings can be made up of low V, med. V & high V so that all strings are = V. The blue LED is used for protection of reversed DUT, device under test. When testing white LEDs, for blue LED might use a white & red LED in series to give about 5 V, which should be safe for a reversed LED. The 470 ohm R could be made from a combination
of series & parallel Rs to give desired test current.

View attachment 108474
View attachment 108474

Tried this and the results are:

supply 12.41v
resistor 463ohms
volt meter reading 1.87v
I tried 23 LED's and results follow:
1 red & 1 green : 1.88v
5 red, 6 green & 3 yellow :1.87v
1 red & 5 yellow : 1.86v

 

Last pass. This one has 4017 Johnson counters for the 7 second and 4 second periods, so both are adjustable in 1 second increments. P1 and P2 are jumper fields for those little 2-pin shunts. The 4017's are wired correctly - the count enable and clock inputs are two inputs to an internal gate, so either can be used for either purpose. U5 is a silicon oscillator from Maxim, $1.25 in ones from Digi-Key. Also, I got lazy decoding the 1 minute period, so it is 64 seconds. Note that this is the lowest body count of all schematics posted, and has no calibration adjustments.

EDIT: updated schematic

ak

 

Mark, some surprises, I did not think 3 different colored LED's would be that close in Vf, that output of 555
@ 6 V Vcc would be so low- 4.38 V @ 20 mA load, & Si diode @ 20 mA would drop .71 V. So for green LED
( 4.38 - 1.87) / .02 = 125.5 ohm, or std. 130. Red LED, ( 4.38 - 1.87 - .75 ) / .02 = 88 ohms or 91 or 100.
AK's TIMER 2 looks OK, just need to stretch beep like in TIMER 3, & brighten up LED's.
Will look at AK's TIMER 3 when I'm feeling better.

 

Olympic 2 was about half concept and half serious. It needs more decoupling, a simple regulator and an output level shifter for the clock source, and the beeper might not be triggered at the correct intervals.

Olympic 3 is farther along. I was thinking of a 9 V source when I designed it, but the oscillator will run on 5 V, so 4.5 V (three AA's) should do it. With that lower voltage the LED resistor should drop to something like 150 ohms. A high efficiency red and low efficiency green should balance out the brightness.

ak

 

Mark, some surprises, I did not think 3 different colored LED's would be that close in Vf, that output of 555
@ 6 V Vcc would be so low- 4.38 V @ 20 mA load, & Si diode @ 20 mA would drop .71 V. So for green LED
( 4.38 - 1.87) / .02 = 125.5 ohm, or std. 130. Red LED, ( 4.38 - 1.87 - .75 ) / .02 = 88 ohms or 91 or 100.
AK's TIMER 2 looks OK, just need to stretch beep like in TIMER 3, & brighten up LED's.
Will look at AK's TIMER 3 when I'm feeling better.

Thanks and get better, take care of yourself dear friend.

 

Hi can anyone help me in building a circuit for a training aid.

It must have a start switch that after pressed gives an audible beep and shows a red led, after a 1 minute delay an audible beep signalling the start of a 7sec delay to end with start of a 4 sec green led(red off) followed back to the red led and restarts the initial beep and 1 min count.

switch -beep- 1min red led - audible beep - 7sec - 4 sec green led (red off) - back to start beep and 1min red led on

It would be a huge advantage if the timings could be programmable.

This product is the closest available to purchase but not quite what I need.
http://www.edinkillie.co.uk/ecatalog/techro-box-p-585.html?cPath=23_25

How easily reprogrammable?
Most trim pots are not designed for excessive use. They get flaky after as few as 100 rotations. Look at the data sheet for the pots you are considering.

 

How easily reprogrammable?
Most trim pots are not designed for excessive use. They get flaky after as few as 100 rotations. Look at the data sheet for the pots you are considering.[/QUOTE

Once set they would remain unless I want to alter the training times but what do you suggest as an alternative?

 

I do not remember of pots being of any concern.

 

I do not remember of pots being of any concern.

Pot's are fine

 

Are Mark Boulton and MarkAB the same person?

 

Are Mark Boulton and MarkAB the same person?

Oh yes sorry don't know why i am in here twice ??? Must be when I go in from my mobile so sorry about this.

 

Oh yes sorry don't know why i am in here twice ??? Must be when I go in from my mobile so sorry about this.

It's not a problem for me; I just wondered.