Suggestions for projects using relays and transistors

The use of relays, bipolar transistors, and power FETs in control and time circuits offers many possibilities to the robotics and mechatronics designer. The following are some ideas for projects using the blocks we have presented so far. Grab your tools and try to create some of these working robots or mechatronics devices:

Design a circuit that can control the movement of two motors in a robot, using sensors with switches.

Design an automatic elevator that can stop at each floor for a few seconds. Use switches or reed switches and magnets as sensors.

Create an automatic door that senses the presence of a person using a switch placed under the carpet. Design it to make some “intelligent decisions” such as not closing if a person is passing through it, etc.


Information about Relays

Radio Shack has some popular relays that are suitable for all the applications described in this book. We selected some of their relays having the right characteristics.

Mini SPDT relay for 2 A currents. This relay needs only 18 mA to be activated and operates in the range voltage range of 7 to 9 V. The Radio Shack part number is 275-005.

Mini SPDT relay for 10 A currents.

This relay has a 12 V × 30 mA coil (400 ?). The part number is 275-248.

Mini DPDT relay for 5 A. This relay needs 60 mA from a 12 V power supply to close its contacts. The part number is 275-249.


Figure 1 shows the pinout for these relays.
Figure 1 shows the pinout for these relays.



A multitude of relay types from other manufacturers can also be found at Mouser Electronics ( to meet any of your design needs.


Information about Transistors

The following tables describe some transistors that are suitable for use as switches in the previously described blocks.





At Mouser Electronics you can also count on a variety of transistor types suitable for load control, including shields and breakout boards with transistors for your microcontroller design project.



N° of component