Introduction: Relay Substitution

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The conventional circuit for actuating a relay transistor (NPN or MOSFET), may be replaced by a Reed relay.


Reed Relay

Step 1: Description of the Concept

Advantages :

  • Reduced number of components
  • Space saving
  • Simplifying installation
  • Connects directly to TTL output
  • Low cost


Limited Switching Power ~ 10W (there are models to 20W).

Technical data:
A reed switch is a magnetic switch, placed in a glass bulb, of which two contacts are made of iron-nickel alloy. In the presence of a magnetic field, contacts magnetize and are attracted to each other.

The maximum input current (TTL output) varies depending on the components between 10 and 16mA. The resistance of the relay coil must be greater than 500 ohms in order to limit to 10mA for a 5V supply.

The maximum cut-off intensity varies between 0.5A and 1A according to the relay model.

The diode is essential. It protects the input device when the current flowing in the relay coil is cut, causing a high voltage to the relay coil.

Relay corresponding to the above specification Reed Celduc D31A3110 (or equivalent PRME 15005, Edr0201 a0500, SIP1A05).

The cost is less than € 5.

Used worldwide microcontrollers (see application project on my site )