# Velleman stroboscope I need a lower duty cycle.

#### Ant1110

Joined Aug 29, 2017
5
I'm a noob but loving circuitry and having a blast learning. I need a strobe for a project. I bought the velleman stroboscope kit and built it, but when I use it for moving things it is blurry. I need the duty cycle to be much lower. I called velleman, watched lots of tutorials on 555 timers and just can not figure out which resistor controls duty cycle. (I know this isn't a 555 but just wanted to learn as much as I could) Velleman also has a 555 kit but doesnt go to 60hz(from my understanding)
This kit has a pot for frequency and a switch for fast (31hz -60hz) and slow (1hz-30hz) Trying to make an optical illusion with spinning objects. I read somewhere that the duty cycle should be less than 1%. Is this kit the way to go if I change out some parts or should I start over with a 555?

Thank you so much for any help.

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#### Alec_t

Joined Sep 17, 2013
11,814
R2 controls the duty cycle, but reducing it significantly may stop the circuit working properly.

#### crutschow

Joined Mar 14, 2008
26,064
You can also try reducing the value of C1.

#### RichardO

Joined May 4, 2013
2,271

#### AnalogKid

Joined Aug 1, 2013
8,828
C1 and R2 set the ON time of the LEDs, independently of the repetition rate set by C2, the switches, pots, etc. Change C1 to 10 uF and see if that is fast enough. It should improve things enough to know you are on the right path.

The on time is set by an equation that has as its heart the product of R2 and C1, called the time constant.

1000 ohms x 0.0001 F (100 uF) = 0.1 s (100 milliseconds). ish. The real number involves the base-emitter voltage of T1, the saturation voltage of T2, whether or not V+ is greater than the reverse breakdown voltage of T1, blah blah blah. But this number relates directly to how long the LEDs are on.

To get a shorter time constant, better to decrease the capacitor than decrease the resistor. R2 sets the base current for T2, and that current can only be so high before it damages the transistor.

ak

#### Ant1110

Joined Aug 29, 2017
5
If I reduce one of the capacitors, wont that make the leds not as bright? Which is going to be dimmer with a short duty cycle anyway. Is this kit a good way to go? or should I start over with a 555? I just need the frequency to be adjustable,(20-60+hz) and the duty cycle very short for rapid moving parts to not look blurry.

I thought this kit would be perfect, but its kinda confusing. Other examples I have seen were very simple.
If I could just switch a couple components this kit would be good.

#### RichardO

Joined May 4, 2013
2,271
C1 and R2 set the ON time of the LEDs, independently of the repetition rate set by C2,...
Thanks. I did not look closely enough at the circuit.

#### Ant1110

Joined Aug 29, 2017
5
Thank you Analogkid, i will read your reply a few times to understand it all. I have seen the equation before, so I will change C1, and play around with the equation as well. I dont want to just change stuff, I want to try and understand why it does what it does. Thank you very much.

#### crutschow

Joined Mar 14, 2008
26,064
Did you mean C2?
No.

Here's the LTspice simulation of the basic circuit showing the LED current pulses, I(D1), for C1 values of 20μF and 100μF:
The pulses still are fairly long.
A 555 circuit may work better if you want shorter pulses than that.

Last edited:

#### Ant1110

Joined Aug 29, 2017
5
That worked great, thanks crutschow and anaogkid. I put a 10uf 16v in, its fast enough to catch without blur.
I had an old dvd player, took it apart and robbed what I needed. It also has a 1uf 50v, I wonder if the 50 volts part would be a problem. The 10 is soldered in already so not going to try but just curious if it would work?
Thanks for all your help everyone.

#### Ant1110

Joined Aug 29, 2017
5
crutschow, could you maybe throw a 10uf 16v and a 1uf 50v in your software, im curious?

#### AnalogKid

Joined Aug 1, 2013
8,828
If I reduce one of the capacitors, wont that make the leds not as bright? Is this kit a good way to go? or should I start over with a 555?
They might not appear as bright, but the change I describe does not alter the LED current, and that is what sets its actual brightness. Starting over with a 555 will not change this. Once you get the pulse width short enough to freeze the motion, and the frequency just right to make things appear to stop, run backwards, or whatever, those two parameters set the driving waveform, and that is independent of whatever the circuit details are.

T increase the LED brightness, decrease R7. Carefully.

ak