Photo Relays: A Beginners Guide

 Relays are a critical part of any design, especially those that require high current to power some of their sections. However, a dilemma typically ensues when choosing between mechanical relays and contactless relays, but that will no longer be the case with the onset of photo relays.

Thus, below you learn about photo relays, what makes them unique, and why they are preferred to mechanical relays.

Read on to find out more.

What are Photo Relays?

 To understand what photo relays are, it is essential you first have a clear understanding of what relays are.

 Put simply. Relays are switches used to provide both high power and low power outputs in electrical circuits.

 Relay models are into two groups;

 Semiconductor relays, also known as contactless relays.

 Electromagnetic relays. Primarily known as mechanical relays.

Now Photo relays are a type of contact-less relays that use light as the impulse signal. Meaning that, unlike the mechanical relays, they do not require contact to switch current to a load.

 Photo relays work by releasing light once the current goes through the LED.

 The released light then falls across the isolation boundary onto the PDA chip light sensor, which powers a connected MOSFET pair.

 Once the MOSFET pair is turned on, AC/DC flows through its power terminals.

Photo Relays versus Mechanical relays

In the table below, features of photo relays are compared against those of mechanical relays.

Photo RelayMechanical relay
Uses LightUses movable contacts
Fast and quiet switchingSlow switching and produces a loud clicking sound
Long-lasting as there are no contact partsShorter lifespan due to pressure resulting from the movable contacts
Low input power consumptionHigh input power consumption

Benefits of Photo Relays

  • Longer lifespan, since they do not have mechanical parts hence slower degradation enhancing their reliability.
  • Photo relays have reduced power consumption.  Relay photo relays require as little as 3 to 5mA level current. To give you a clear perspective; contact relays consume about 100 mW, whereas photo relays consume 0.5mW
  • Smaller due to their package style.
  • Faster switching speeds.
  • Affordable as they don’t require mechanical relay replacements.
  • Enhance device functionality. 

How to make a portable photo relay

Items you will need to make a portable photo relay.

  •  Transformer
  •    NE-2 Neon Lamp
  •   Plastic Case preferably 12cm*8cm *3cm
  •   Light-dependent resistor
  •   A receptacle
  •   7555 CMOS timer
  •   A solid-state relay; item model 2 Amp
  •   Tip Jack
  •   Bottle top
  •   Silicone rubber Sealant
  •   Tip Jack
  •  LED mounting Sleeve


 Place the SS relay, LDR leads, and the transformer onto the plastic case. Using silicone rubber sealant, secure them on the plastic.

 Place the small parts such as the diode, capacitor, and integrated circuit onto the circuit board.

Circuit Board 

Circuit Board 

 After which, mount the circuit board onto the plastic case. Place the circuit board between the SS relay and the transformer next to the LDR leads.

 Now connect the 2 pin header onto the LDR.

 Wire the SS relay to connect to the circuit board, and then wire the transformer to connect to the circuit board.

Place the output receptacle onto the SS relay while installing the transformer wiring and inputting the line cord.

Finally, install the neon lamp and ground terminal connecting it to the transformer. Your portable transformer is now complete.

Below is a circuit diagram to provide direction:

 It is worth noting that there are three variations of a photo relay, and they include:

  •   240 VAC single pole version
  •  Weatherproof version
  •   High current Version

Let’s have a closer look at each and learn how you can make them. 

240 VAC single Pole Version

Making the 240 VAC single pole versions is relatively easy. All you have to do is use a 230V/6v×2 transformer in place of the 240VAC and remove the neon lamp and resistor (150k)



Weather Proof Version

By removing the light sensor using low voltage wiring, you will have created a weatherproof Photo relay version.

However, you will need to enclose part of the Light sensor in a translucent waterproof tin to place the remaining circuit parts in a covered location. As shown by the circuit diagram below

High Current Photo relay Version

For a high current photo relay version, you will need to replace the original resistors. That you used when making the initial photo relay with more powerful resistors to handle the high current.

This system is preferred as it provides enhanced nighttime security.

Similarly, you can follow the circuit diagram as shown below to pilot a 240VC relay.

Safety Precautions to follow when making a portable photo relay

 Whenever dealing with any digital device, you must adhere to safety regulations to prevent you and the electronic item from harm or damage.

 Some of the precautions include: 

1.      Work on a dry surface and environment to avoid electric shocks.

2.      Ensure you have turned off any power source linked to the device before working on it. You should also disconnect all the wiring attached to the photo relay.

3.      Conduct a polarity check to ensure all the conductors are de-energized. A polarity test will also prevent your device from short-circuiting and also improve your device functionality

4.      Only use a voltage that the system can handle as a higher voltage may lead to an explosion once you connect the photo relay to power.

5.      Double-check the wiring on your digital device to ascertain you linked it correctly before connecting the photo relay to a power source.


 With the continuous advancement of technology, designers are looking for ways to develop quality relay systems.

Relay systems that can handle their tasks with ease, be durable, and be cost-effective.

Thus they are moving away from mechanical relays.

Given the popularity and stellar reputation that photo relays enjoy, it is safe to say that they are the future.

Well, that concludes the article, don’t hesitate to contact us for any inquiries you might have, as we are more than happy to help.

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Emma Lu
Our professional engineering support saves our customers a lot of trouble and loss. >>>>>> After you place the order, our engineer will conduct technical reviews to make sure the parts can be mounted well/correctly on the boards. We will check if the component packages match well with the Gerber footprints, if the part numbers you provided match well with the descriptions, and if the polarity is clearly marked. >>>>> When your design is ready, please send your Gerber and BOM so we can quote and start!