LED drive circuit with transistors (h bridge)

Hi, I'm a student working on building a pulse oximeter, making use of a msp430 LaunchPad. I'm trying to implement a LED drive circuit right now, but it doesn't work as expected. I've attached the diagram, which is taken from the TI paper **broken link removed**.

Here's the circuit design.

Connecting a 3V input to the emitter pins of the pnp transistors, I will not be able to get either LED on at all.

Removing the voltage at the emitter pins of pnp transistors, and connecting pin 5 of DAC to a voltage of above 3V causes the infra red LED to light up. Red LED then lights up if I switch this >3V supply to pin 10 of DAC. BTW, there is no DAC in my breadboard, I am just using a wire with supply voltage. Why is it working in an opposite way, isn't the IR LED supposed to light up when pin 10 has voltage, and red LED on when pin 5 is on?

I also connected the P1.0 and P1.6 to the MSP430, with coding such that the LEDs blink alternately, and it does work when I placed jumper pins to on-board LEDs. Vcc for msp430 is 3V.

Transistors used are C557B for pnp, 547C for npn. I just am not able to figure out why it is not working as it should, would appreciate any help possible ..

To “open” NPN transistor working as a switch you have to apply positive (with reference to the Emitter) voltage to its base (UBE>=0.65V).
To “open” PNP transistor working as a switch you have to apply negative (with reference to the Emitter) voltage to its base (UBE=<-0.65V).

To operate IfraredLED you have to drive DAC (10) HIGH AND P1.0 LOW ( AND DAC (5) must be LOW AND P1.6 must be HIGH)
To activate VisibleLED you have to drive DAC (5) HIGH AND P1.6 LOW (AND DAC (10) must be LOW AND P1.0 must be HIGH).

Only when you fulfil conditions described above the LEDs will operate as intended.

:wink:
IanP