email@example.com wrote on 8/4/2017 11:03 AM:
A 555 timer is an excellent idea to generate a control voltage that ranges through 1/3 Vcc and 2/3 Vcc. If you are working with Vcc = 5 volts, 1/3 will be 1.7 volts or a bit over two diode drops. You can use a diode in series with the base of an NPN transistor. Then the two diodes will drop about 1.4 volts giving you about 0.3 volts residual on the emitter. Connect the LED between Vcc and the collector and a resistor between the emitter and ground.
At max control voltage the resistor will have 2/3 Vcc - 1.4 volts or 1.9 volts on the resistor. This will set the maximum current, about 10 mA is good. So a 200 ohm resistor would do nicely. Minimum current then is 1.5 mA which may not be enough of a contrast. Adding a Schottky diode to the base would drop the minimum current to nearly zero.
Use a circuit like the 555 to generate a square wave as fast as you want the LED to vary between getting brighter and dimmer. Ten Hz may be too fast to see, unless you want it for the strobe effect.
Integrate the square wave with a series resistor and a capacitor to produce a triangle wave.
Use that to drive the LED from a transistor through a resistor, connected to the supply voltage. You will need to work out the details, but you may find a 555 circuit that can generate the triangle wave directly.
I missed the bit about the LED needing up to 3.7 volts. That would make the brightness top out a bit early with a 5 volt supply. If you can't use a higher Vcc, you can add a resistor to the base circuit to scale down the triangle wave voltage so the 2N2222 doesn't quite saturate.
On Friday, August 4, 2017 at 8:03:25 AM UTC-7, firstname.lastname@example.org wro te:
make a 20 mA UV LED brighten and dim slowly at about 5 or 10 Hz (LED needs
3.5-3.7 V). Any suggestions? Only thing I could think of was some kind of 555 circuit but that is just due to my limited experience.
Well, yeah, a '555 can do it. If you're running from +5V, just ground the cathode and give the UVLED a few milliamps current source (I'm thinking thirty;; we'll cut it down soon). Set the '555 up as a square wave oscillator, and connect an NPN transistor base to the timing capacitor, emitter through 100 ohms to ground. Then the transistor collector will steal ten to 25 mA from (you guessed it yet?) th e anode of the LED.
Those values assume a +5V power supply, and give a roughly triangle-wave mo dulation of the LED. The resistor/capacitor for the '555 will have to deliver base current to the transistor (so should be low-ish impedance, like 2k ohms).