0
\$\begingroup\$

I have a little BLE device that can drive an LED at variable brightness (according to a programmable 0-255 byte value). This works, but has the major caveat that it can only source 5 mA, whereas many LEDs are spec'd to run at 20 mA nominal. The result is that it can only drive the efficient tiny SMD chip LEDs, and not most discrete LEDs.

So I'm looking to make a little current amplifying circuit to take the 0-5 mA and amplify it to the range of 0-20 mA. What approach would be the best to go about doing this? Any pointers are greatly appreciated.

\$\endgroup\$
8
  • \$\begingroup\$ What's the voltage of the IO pin, and what other voltages do you have in the system? \$\endgroup\$
    – pipe
    Oct 23 '17 at 19:00
  • 1
    \$\begingroup\$ You have to show the actual schematic of what you're doing. I doubt that you have 0 - 5 mA coming out of the BLE device but that you're actually using PWM to dim the LED. In that case you do not need a current amplifier circuit, what you need is simply a MOSFET that is controlled by the PWM signal. That MOSFET then drives the LEDs. There are plenty of questions to be found on that subject on this site. Also, specify which BLE device you're using. All in all this question is too vague to answer properly. \$\endgroup\$ Oct 23 '17 at 19:07
  • \$\begingroup\$ The device in question is an RFduino RFD22301. The way it's set up to drive LEDs is via the GPIO lines, so that does sound like PWM. The device specs say that it can source at most 5ma per GPIO line, and can drive at most three at 5ma at the same time. General Purpose I/O (GPIO) input high voltage 2.1VDC. Output high drive current 5ma. Spec sheet: link \$\endgroup\$
    – tansvaal
    Oct 23 '17 at 19:22
  • \$\begingroup\$ So are you directly hooking the LED between the GPIO pin and ground, currently? \$\endgroup\$
    – jonk
    Oct 23 '17 at 19:30
  • 1
    \$\begingroup\$ Before you build the booster circuit with brighter leds, don't connect the GPIO directly to led and the led to ground. It will drive all the GPIO output. You should always put a resistor between the led and the ground, or between the GPIO and the led, every two solutions does the same. \$\endgroup\$
    – mguima
    Oct 23 '17 at 19:46
0
\$\begingroup\$

You need add this to your circuit, see picture below. You can power this addition with 3V (instead of 5V as show in picture), maybe you'll have to adjust the R2 value.

You can learn more about it in the following link: https://learn.sparkfun.com/tutorials/transistors/applications-i-switches

There's an animated GIF at this page that is very good to help you understand the concept.

enter image description here

\$\endgroup\$
14
  • \$\begingroup\$ Thx, but that looks like an LED switch circuit, not a current booster circuit. Or am I overlooking something? \$\endgroup\$
    – tansvaal
    Oct 23 '17 at 20:21
  • 1
    \$\begingroup\$ @tansvaal, the current to drive the LED comes from the +5 V supply, not the control GPIO. So in that sense it is a current amplifier. \$\endgroup\$
    – The Photon
    Oct 23 '17 at 20:26
  • \$\begingroup\$ If the current comes from the +5 V supply, then how to use the PWM signal to control the current from 0-20ma? \$\endgroup\$
    – tansvaal
    Oct 23 '17 at 20:31
  • \$\begingroup\$ So if I connect the GPIO PWM signal to the "CONTROL" gate of Q1, then by varying the PWM signal it will have the effect of running the LED from 0 - 100% brightness (with the additional current provided by the +5 V supply going through the limiting resistor). Is that essentially correct?. \$\endgroup\$
    – tansvaal
    Oct 23 '17 at 20:40
  • \$\begingroup\$ Yes, although the scale is not linear, i.e. 50% PWM is not 50% brightness. This is not usually important. \$\endgroup\$
    – BeB00
    Oct 23 '17 at 20:52
0
\$\begingroup\$

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.