Several simple, novel and practical 850 Darlington transistor application circuits

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The U850, also known as the 850, is a medium-power NPN-type high-gain Darlington transistor that has recently entered the domestic market. It boasts impressive electrical characteristics such as BVCEO ≥ 35V, ICM = 1A, PCM = 2W, β = 1000–10000, and ft = 180MHz. The device is available in two package types, with its physical appearance shown in Figures (a) and (b), and its internal structure illustrated in Figure (c). Due to its superior performance, the U850 is widely used not only in imported video recorders and automatic control systems but also in common consumer electronics. Below are several practical and innovative application circuits using the U850.

1. 350mA Regulated Power Supply

This regulated power supply circuit uses just one U850 as the adjusting tube. Thanks to the Darlington configuration inside the transistor, it functions similarly to a complex compound series regulator. It is ideal for use in electronic keyboards, tape recorders, and game machines. By adjusting the voltage regulation value of the 2CW voltage regulator, you can achieve different output voltages ranging from 5V to 9V.

2. Toxic Gas Alarm

Toxic gases like carbon monoxide (CO) pose serious health risks. At concentrations as low as 0.02% (200ppm), CO can cause headaches after 2–3 hours of exposure. At 0.04% (400ppm), it may lead to dizziness, and at 0.08% (800ppm), unconsciousness can occur. Therefore, having a reliable gas detection system is crucial for safety.

This circuit uses the U850 along with a MQK-2 gas sensor to detect harmful gases. In normal air, the sensor has high resistance, keeping the base of the U850 low. When exposed to toxic gases, the resistance drops, causing the base voltage to rise and triggering the U850 to activate an alarm IC. This setup can be modified to control relays, turning into an automatic exhaust system when connected to devices like kitchen fans.

3. Voice-Controlled Switch Circuit

This voice-activated switch uses the U850 to amplify sound signals captured by an electret microphone. The amplified signal triggers an IC-based voice control circuit, which toggles a relay to switch on or off connected loads. Adjusting R2 allows fine-tuning of sensitivity, making the system responsive to claps or voices from about 3–4 meters away.

4. Mini AM Radio

A compact AM radio using the U850 simplifies the design. Unlike traditional setups requiring additional amplifiers, the U850 can directly drive low-impedance headphones, making the circuit more efficient and easier to implement.

5. Ultra-Range FM Wireless Microphone

A wireless FM microphone built with the U850 and a D40 transmitter offers a transmission range of up to 1.5 kilometers. It’s ideal for school radio groups, model aircraft, security monitoring, and even small-scale broadcasting in remote areas. With two transmitters and receivers, you can create a simple intercom system for long-distance communication.