What is the difference between a photocell and a phototransistor?

Hey there! As a photocell supplier, I've gotten tons of questions about the difference between photocells and phototransistors. So, I thought I'd break it down for you in a way that's easy to understand.

Let's start with the basics. A photocell, also known as a photoresistor or light - dependent resistor (LDR), is a simple and cost - effective device. It's made of a semiconductor material whose resistance changes depending on the amount of light it's exposed to. When there's more light, the resistance of the photocell decreases, and when it's dark, the resistance goes up.

On the other hand, a phototransistor is a bit more complex. It's a type of transistor that's sensitive to light. Just like a regular transistor, it can amplify electrical signals, but it uses light as the input signal instead of an electrical current. When light hits the base region of the phototransistor, it generates electron - hole pairs, which in turn allows a current to flow between the collector and the emitter.

How They Work

Photocell Operation

The operation of a photocell is pretty straightforward. When photons from light hit the semiconductor material in the photocell, they provide enough energy to free some electrons from their atoms. These free electrons increase the conductivity of the material, which means the resistance drops. You can think of it like a traffic jam on a road. In the dark, there are few cars (electrons) moving, so it's hard for traffic (current) to flow. When the light comes on, more cars (electrons) are on the road, and traffic (current) can move more easily.

Phototransistor Operation

A phototransistor works a bit differently. It has three layers: the emitter, the base, and the collector. When light strikes the base region, it creates electron - hole pairs. These pairs cause a small current to flow into the base. Just like in a regular transistor, this small base current controls a much larger current flowing between the collector and the emitter. It's like a small key that can unlock a big door. The amount of current flowing through the phototransistor depends on the intensity of the light hitting it.

Sensitivity and Response Time

Sensitivity

Photocells are generally less sensitive than phototransistors. They can detect changes in light levels, but they're not as precise. For example, if you want to detect a very faint light or make very accurate light - level measurements, a phototransistor would be a better choice. A phototransistor can respond to very small amounts of light and can provide a more linear output with respect to the light intensity.

Response Time

The response time is another key difference. Photocells have a relatively slow response time. It can take them several milliseconds to change their resistance in response to a change in light. This is because the process of freeing electrons in the semiconductor material takes a bit of time. In contrast, phototransistors have a much faster response time, often in the microsecond range. This makes them suitable for applications where you need to detect rapid changes in light, like in optical communication systems or high - speed light sensors.

Applications

Photocell Applications

Photocells are used in a wide range of applications where a simple and cost - effective light sensor is needed. One common use is in streetlights. When it gets dark, the photocell's resistance increases, and this change in resistance is used to turn on the streetlight. They're also used in automatic garden lights, camera light meters, and some simple light - controlled switches. You can check out our Twist Lock Photo Control for a great example of a photocell - based product. It's easy to install and works well for controlling outdoor lighting.

Phototransistor Applications

Phototransistors are used in more advanced applications. They're commonly found in optical encoders, which are used to measure the rotation speed and position of motors. They're also used in infrared remote control receivers, where they can quickly detect the infrared signals sent by the remote control. Another application is in fiber - optic communication systems, where they can detect the light signals transmitted through the fiber. If you're looking for a high - performance light sensor for such applications, our 12v Dc Photocell Sensor might be a good option.

Cost

Cost is always an important factor when choosing between a photocell and a phototransistor. Photocells are much cheaper to produce. They have a simple design and use relatively inexpensive semiconductor materials. This makes them a great choice for applications where cost is a major concern, like in consumer products or low - end lighting control systems.

Phototransistors, on the other hand, are more expensive. Their more complex design and the need for precise manufacturing processes drive up the cost. However, if you need the high sensitivity and fast response time that they offer, the extra cost might be worth it.

Output Characteristics

Photocell Output

The output of a photocell is usually a change in resistance. To use this output in a circuit, you typically need to convert the resistance change into a voltage change using a resistor in a voltage - divider circuit. The relationship between the light intensity and the output voltage is non - linear, which means that the change in voltage is not directly proportional to the change in light intensity.

Phototransistor Output

A phototransistor provides a current output. This current can be easily converted into a voltage using a load resistor. The output of a phototransistor is more linear with respect to the light intensity, which means that the current changes in a more predictable way as the light intensity changes. This linearity makes it easier to use in circuits where you need to make accurate light - level measurements.

Power Consumption

Photocells generally consume very little power. Since they're just a passive device that changes its resistance, they don't require an external power source to operate. They only draw a small amount of current when they're part of a circuit.

Phototransistors, on the other hand, do require a power source to operate. They need a voltage to be applied across the collector and the emitter to allow the current to flow. This means that they consume more power than photocells, especially when they're operating at high light levels.

In conclusion, both photocells and phototransistors have their own unique features and are suitable for different applications. If you're looking for a simple, cost - effective, and low - power light sensor for basic light - control applications, a photocell is a great choice. But if you need high sensitivity, fast response time, and more accurate light - level measurements, a phototransistor would be the way to go.

If you're interested in purchasing photocells for your projects, don't hesitate to reach out for a procurement discussion. We can help you find the right product for your specific needs.

References:

• "Electronic Devices and Circuit Theory" by Robert L. Boylestad and Louis Nashelsky
• "Optoelectronics: An Introduction" by J. Wilson and J. F. B. Hawkes