Power-outputs for the H0420 programmable MP3 controller/player

The H0420 programmable MP3 controller/player has 16 general purpose digital I/O pins. These pins can be programmed (of course) to be either input or output, and they adhere to TTL signal levels. When configured as outputs, each pin can furnish up to 20 mA, sufficient to directly drive a LED or an opto-coupler.

The I/O pins of the H0420 are protected against short-circuit with an internal 220Ω resistor. When a yellow LED is branched at an output, the current flowing through that LED will be approximately 12 mA (the forward voltage of the yellow LED is approximately 2.2V, so the voltage across the internal resistor is the high level of a TTL gate minus 2.2V, or rougly 2.6V; this results in a current through the resistor of 12 mA). Other colours of LEDs have different forward voltages and the current flowing through these LEDs varies accordingly.

When the H0420 must drive something that has a little higher power requirements, either current or tension, you have several options ranging from analog switches, to (solid-state) relays. A simple, effective and cheap way, though, is with a circuit consisting of a single power transistor and a resistor.

The above schematic shows the simplicity of the circuit. The power transistor is in a typical "common-emitter" configuration. Although the schematic shows a BD437, many power transistors are suitable (e.g. 2N5190). The 1k resistor limits the current through the base of the transistor. If you use a different transistor, you may need to adjust the 1k resistor.

The left side of the schematic connects to any of the I/O pins of the H0420. The right side can switch a tension of up to 20 V and 4 A (but the transistor needs a cooling radiator for such high currents). This circuit is almost too simple to build a PCB for, and therefore it has often been constructed right on top of a crimp connector. In the figure below, the circuit is connected to I/O pin 15 and its associated ground pin. These two pins are at connector J7 of the H0420 MP3 controller.

Further reading & references

H0420 Programmable MP3 Player

The main page for the MP3 player that this article applies to. Many of the application notes use the digital I/O lines to drive LEDs or other electronics.