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Old 1st Apr 2019, 3:28 pm   #62
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: Turntable Stroboscope

A 555 might be relatively easy, but the elegant way would be to do it in the counter logic.

You have a chain of flip-flops producing 300Hz, so four flip flops before the one whose output you're using will be at sixteen times your 300Hz. So one period of this ON followed by 15 periods OFF is what you want for your LED.

So we want a 4-bit binary counter in 74HC logic so let's look at 74HC161 about £0.30 isn't going to hurt the pocket and its in a 16pin through-hole package.

So now I need a datasheet.

Its pins are:

CEP Count Enable Parallel. Don't want this, we aren't loading data into it. tie to ground
CET Count Ensable Trickle input. Tie this to 5v we want to turn this function on
CP Clock pulse. yes we want this, feed in 4.8kHz from your divider chain to clock this counter.
Not(MR) Master reset, active low. Tie to 5v we don't want this active
Not(SR) Synchronous Reset. Tie to 5v, we don't want this reset either.
P0 to P3 We're not using the data inputs, tie them all to ground (never leave CMOS inputs floating)
Not(PE) we don't want to do parallel loading so tie to 5v to disable
Q0 to Q3 these are the flip flop outputs. We don't use them but must let them be free to move, so don't connect.
TC Terminal Count This is the one we want! It should pulse at 300Hz for us and give a pulse exactly 1/16th of a cycle.

So in pin numbers
1 Not(MR or SR) tie to 5v
2 CP this is out input. feed in 4.8kHz logic level signal.
3, 4, 5, 6 are P0-P3 so ground them.
7 is CEP tie to ground
8 is the ground pin, so you guessed it, ground it.
9 is Not(PE) so tie to 5v
10 is CET tie to 5v
11, 12, 13, 14 are the Q outputs, leave unconnected
15 is our output (Hooray!)
16 is the power supply. Tie to 5v and it's good practice to add a 0.1uF capacitor to ground.


THis might get you going for about 30p and that 0.1 ceramic decoupling capacitor.


David
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