Overview
Its bright enough to use without dimming the room lights and with the aid of a frequency meter or oscilloscope, can measure rotational frequency to within a few degrees per second. This LED stroboscope is a fairly simple circuit for the similarly simple task of measuring the speed of things that rotate. I wondered how fast some DC fans I had on the bench were, and tried a quick-and-dirty LED driver, driven by pulses from a function generator. I was pleased to see that, in contrast to my earlier experience years before, the white LEDs I have now, BUXC333 from BestHongKong.com, is bright enough to use with the room lights on.  I decided to go ahead and build up a small stroboscope with one of them.

The circuit and an optional 9 volt radio battery are housed in a small plastic box. A range switch selects the RPM range,  and a knob adjust the oscillator frequency. A white LED is mounted at the end of about 60 cm of wire, so it can be aimed at whatever it is that I want to illuminate with it, without affecting the orientation of the box.

When operating in the hand-held mode from its internal battery, the pot is adjusted so that the LED flashes once per rotation of whatever is being measured, and the revolutions per minute are estimated by reading from the hand calibrated dial.

A headphone jack mounted in the box provides connections for an external power supply, such as a bench supply or a "wall wart" power supply and an pulse output so that the LED pulse frequency can be accurately measured with a frequency meter or oscilloscope.

A note of caution: Be careful not to load the pulse output, since its not buffered and loading it can stop the oscillator.

Circuit

The circuit is composed of an oscillator, a pulse slicer, and a gated constant current source. All of the circuits except the anode of the LED receives power from an LM78L05 5 volt regulator. 

The comparitor connected to pins 1,2, and 3 is connected as a 50% duty cycle oscillator. This circuit's oscillation frequency is = 1/(1.4 R C) where R is the sum of of the 10k fixed resistor and the 100k pot, and C is the capacitance from pin 2 of the comparitor to ground. Multiplying the frequency in Hertz by 60 gives RPM (Revolutions Per Minute), so the formula becomes RPM= 43/(R C).  Using this formula, you can choose a value for the capacitor that will center the oscillator's range around the frequency you are most interested in.

The second comparitor, the one using pins 5,6, and 7, slices the sawtooth that appears across the oscillator capacitor. Whenever the voltage of the sawtooth exceeds the voltage on the wiper of the 50k pot, the comparitor's output is high. When the comparitor's output is high, the constant current source made of the two transistors is on, and the LED emits light.


Whenever there is current available through the 3.3k resistor, the upper 2N2222's current is limited to that which provides enough voltage across the 10 Ohm emitter resistor to turn on the lower 2N2222.  This voltage is about 600 millivolts, so current through this resistor, which is approximately the same as that of the LED is set to about 60 milliamps. That's twice the datasheet's 30 milliamps maximum current, but well less than the 100 mailliamps peak pulse current. To reduce the current, make the emitter resistor larger. To increase the current, make the resistor smaller.

Assembly and Calibration

I had a pretty good idea of what I was going to use as a circuit, so I went ahead and modified the box - cutting the holes and mounting the frequency control pot, the on/off switch, range switch, and I/O jack, and cutting the circuit board to size before prototyping the circuit. If I had built the circuit first, I think I would have used a larger box. The small parts were mounted on a small piece of pad-per-hole phenolic circuit board and the leads were bent toward one another and soldered together.  This layout is not critical, but you should keep the leads to pins 2 and 3 of the comparitor as short as practical since noise pickup on these leads can affect the oscillator.

After you've got it all connected together test the circuit using an inexpensive red LED rather than an expensive white LED because if the constant current source doesn't work, or if the duty cycle pot is turned the wrong way, you run the risk of "frying" the LED. 

Set the duty cycle pot by aiming the LED at a rotating target, preferably one with a timing mark of some kind on it, like the small fan blade with a piece of white tape shown in the photograph at the top of this page, and adjust the duty cycle pot for the best tradeoff between apparent brightness and sharpness. A large duty cycle will smear the image. A very short duty cycle will look dim. The tradeoff is yours to make.

You can also use an oscilloscope to set duty cycle by probing pin 7 of the comparitor and adjusting the positive going pule to the desired duty cycle. I recommend 10% as a starting point.

You can calibrate the frequency adjust knob if you want. I put a piece of white cardboard on the front of the box and drew a scale on it with a ball-point pen. Using a frequency meter to monitor the oscillator output of the headphone jack, I made marks at dial positions corresponding to 500, 750, and 1000 RPM on the slow scale, and at 300, 3000, 4000, 5000,  and 6,000 RPM on the fast scale. With this scale, I can estimate revolutions per minute when operating in the battery powered hand held mode.

              
The component side of Guillermo's board is shown above.



Above is the copper side of Guillermo's board.




Guillermo used external pots to control both the frequency and the duty cycle. Other wires
lead to the on/off switch, the fast/slow switch, and a connector for the 9 volt battery.

 

Guillermo's stobe (above) used the same circuit as the origingal single LED stoboscope,
but it drives seven LEDs in parallel. This gave him the same amount of light as the single LED strobe,
but with the advantage of the illumination being more even.