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Design Notes - 38khz emitter

Posted by admin on February 11, 2013

Notes on the MSP430-based IR transmitter driver.

We are using this transmitter:


from Newark.

Forward voltage is 1.3-1.7 V, nominal forward current is 100ma.

We can't really drive these in pairs from the 3.3V supply; I have been pondering a design where the microcontroller is driven by a CR2032 coin cell and the oomph for the LEDs comes from an auxiliary battery pack that can both be monitored by the microcontroller (with status displayed) and that can have a higher drive voltage as well. 

I have in hand a 5V LDO regulator (MCP1827-5002E/AT). We can drive this from four 1.5V AA cells and have the minimum 5.6V supply for the LDO, and drive sets of pairs of LEDs. We get a bonus where we can monitor the PWRGD line from the regulator, as well as make use of the SHDN input to do microcontroller control of the LED drive. At some point I would like to include a radio transceiver on the LED driver module as well as the detector, for overall system control and monitoring.

Anyway, if we're driving LEDs with Vf = 1.3V from a 5V supply, we can drive strings of three (3.9V, leaving a 1.1V margin away from supply. However if the Vf were closer to 1.7 we might be in trouble. TEST1: measure the Vf on our LEDs).

To drive If=100mA through them, we have Rled = (5V - 3 * 1.3V) / 0.1 = 11 ohms. Closest standard resistor is 10 ohms, making our current 110mA, a bit higher than our target. If the LEDs' Vf is 1.5V, the current would be 50mA.

Current 0.1        
Vcc # leds Vf R R - chosen I
5 2 1.3 24 22 109.1
5 3 1.3 11 10 110
5 2 1.5 20 22 90.9
5 3 1.5 5 25 20
5 2 1.7 16 16 100
5 3 1.7 XXX