Several simple, novel and practical 850 Darlington transistor application circuits

The U850, also known as the 850, is a high-gain NPN Darlington transistor that has recently entered the domestic market. It features impressive electrical parameters such as BVCEO ≥ 35V, ICM = 1A, PCM = 2W, β ranging from 1000 to 10000, and ft of 180MHz. The U850 comes in two package types, shown in Figures (a) and (b), with its internal structure illustrated in Figure (c). Due to its excellent performance, it is widely used in both imported video recorders and common electronic circuits. Here are some practical and innovative application examples.

1. 350mA Regulated Power Supply

This regulated power supply uses only one U850 as the adjusting tube. Thanks to its Darlington configuration, it functions like a complex compound series regulator, making it ideal for use in devices like keyboards, tape recorders, and game machines. By adjusting the voltage regulator in the circuit, you can obtain output voltages between 5V and 9V.

2. Toxic Gas Alarm

Toxic gases like carbon monoxide (CO) can be extremely dangerous. Even low concentrations can cause health issues or worse. This circuit uses the U850 in combination with a gas sensor (MQK-2) to detect harmful gases. When the gas concentration rises, the resistance drops, triggering the alarm. It can be adapted into an automatic exhaust system by using a relay instead of an alarm IC.

3. Voice-Controlled Switch Circuit

This voice-activated switch uses a U850 as a preamplifier and works with an IC-based voice control circuit. A microphone captures sound, which is amplified and sent to the IC to trigger a relay. Adjusting R2 helps control sensitivity, allowing users to activate the switch by clapping or speaking at a distance of about 3–4 meters.

4. Mini AM Radio

Building a compact AM radio can be simplified using the U850. Instead of requiring two regular transistors, this circuit uses the U850 to drive headphones directly, reducing complexity and component count.

5. Ultra-Long Range FM Wireless Microphone

This FM wireless microphone, built using a U850 and a dedicated FM transmitter, offers an effective range of up to 3 kilometers in rural areas. It's useful for school radio, model aircraft, security systems, and even basic broadcasting. With two units, you can create a simple intercom system.

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