Circuit diagram for driving LED with logic signal levels

Shown in the figure is an example of a circuit that drives an LED with logic signal levels. In the circuit, TC4069 is used to form a Schmitt circuit, and the rising characteristic of the output pulse waveform obtained is steeper than that of the input signal. Therefore, even if the input signal is an analog signal, better switching characteristics can be obtained. In Figure (a), the LED driving circuit adopts VT1 and VT2 Darlington connection to improve its driving capability. In Figure (b), the current flowing through the LED is about 20mA. The TLR-102A light-emitting diode with very bright brightness is used here. A current of 3mA can make it have sufficient brightness. Therefore, the driving circuit does not need to use a Darlington tube. , just use 1 transistor. In the circuit, three inverters are used, among which the first two inverters constitute a Schmitt circuit, and the latter stage is only a buffer.

Compared with the circuit in Figure (b), the LED switching speed is faster, but the power consumption is larger. Because when the switching transistor VT1 is on, a current I1 flows, and when VT1 is off, a current I2 flows, that is, when VT1 is on and off, current flows, so the conversion efficiency is low. This circuit is suitable for optical communication and infrared remote control devices.