How to turn on LED with only a cathode pin

Question

I'm looking at the datasheet for some TIL311 hexadecimal displays which I ordered (but have not yet arrived. (Actually they appear to be TIL311 clones, but should be compatible.))

According to the datasheet, the left and right decimal point LEDs have their cathodes exposed via pins 4 and 10. The anodes are not connected to a pin; I assume they are hardwired to Vcc within the IC.

I want to turn the decimal point ON when I have +5v on a certain wire.

Am I correct in thinking that the right way to do this would be to use a discrete transistor to allow current to flow from the cathode to ground when it (the transistor) turns on? If so, what's a good kind of transistor to use?

Or is there a simpler way that I haven't thought of?

Answer 1

You could use a low-side NPN transistor (e.g. 2N4401) or N-channel MOSFET (e.g. 2N7000). Connect the dot pin to a resistor (~1kOhm), and connect the other end of the resistor into the switch's collector/drain. Connect the switch's emitter/source to ground and feed an on/off signal to the base/gate through a resistor (say 10~100 Ohms).

Unless you want it always on -- then you could just connect a resistor to the dot pin to ground (again, ~1k Ohm).

Answer 2

The datasheet schematic and description tell you all you need to know. The anodes of the decimal point LEDs are connected to the supply and you need to provide current limiting. The other LEDs are controlled at 5mA more-or-less. I do not see a spec for decimal point forward voltage, but it should be about 1.8V for a red LED, so a resistor of about (5V-1.8V)/0.005A = 640 ohms (use 680 as the next highest standard value) should be fine. With 5mA collector current, your base should get something like 1/20 of that, so that's 250uA. Vbe is about 0.7V so a base resistor of (5V-0.7V)/250uA = 17.2K (you can use 10K or 15K) will work well.

^{simulate this circuit – Schematic created using CircuitLab}

When the input is at 5V, the 280uA or so base current turns Q1 on, 5mA passes through the decimal point LED and it illuminates.

Answer 3

Read carefully the datasheet. The info you are looking for is there.

Therefore the two decimal point LEDs are connected between pin 1 (LED supply, common for both) and the two cathode pins (4 and 10).

Then it's up to you how to power them up. If you want to control them independently, you just wire pin 1 to a suitable positive power rail and put any switching device between each cathode and ground. It may be a BJT, a MOSFET or even a relay. If you want to control them via a MCU GPIO pin, you can also connect it to that pin(s), as long as the MCU can sink enough current (of course you must add a limiting resistor in series).

Since the datasheet reports a 5mA recommended operating current for DPs:

you may be confident that many modern MCUs can sink that much current (for example a GPIO pin of the ubiquitous Arduino boards, from its ATmega328P MCU, can handle it without problem).

For example:

drive the GPIO pin LOW to light up the decimal point whose cathode is connected to pin 4 of the display.