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Introduction  Description  Construction  Testing  User Manual

Note: this project has been discontinued.

Note: TEMP08 no longer supports RELAY05 with current versions of software.

Build a
Relay Interface for the TEMP08: the RELAY05


The RELAY05 is an adjunct kit, originally designed for use with the Midon Design TEMP05 and TEMP08 modules.  With both units connected up to your PC, 8 relays can be controlled via the PC's serial interface.  Of course, the TEMP05/08 unit is not required to operate the relays.  Any TTL-level signals can be connected to the RELAY05 U1 connector to activate the relays.  The unit operates from 12VDC at about 700mA with all relays activated.

The relays used in this project are rated to 10A at 30VDC or 12A at 250VAC.


Figure 1. RELAY05



Figure 2 RELAY05 Schematic Diagram

As you can see from Figure 2, this is a fairly simple "step and repeat" circuit, utilizing the same basic relay drive circuit for the 8 relays.  The transistors, Q1-8, drive the relays from the control signals presented at the inputs from U1.  Diodes D1-8 are used to prevent EMF back-surge from the relay de-activation from harming the transistors.  The same transistor drives the LED's through current-limiting resistors R1-8.  The relay outputs are brought out via terminal strips J1-8.

The "D" version PCB also includes space for installing a terminal strip, full-wave bridge and capacitor for independent powering, should that be required.


Standard construction techniques may be used for this circuit and a printed circuit board is available if desired.

If building the RELAY05 from the PCB, refer to Figure 3 for component placement. Begin by inserting an IC socket for U1.  This will be used to inter-connect with the TEMP05 module. Follow this with the insertion of all passive components.  Follow this by inserting the LED's, diodes, and transistors.  Insert the relays and terminal strips last.

There is only 1 type of discrete resistor; 1.0K.  The color codes for it are shown in Table 1.  Insert the resistors where shown in Figure 3.  The DIP resistor, R9, should be mounted as shown, with care to ensure pin 1 is oriented to the square pad on the PCB.  Alternatively, discrete 10K ohm resistors could be used in it's place.  Use the alternate pads beside R9 should you choose to mount individual resistors.

Resistor Color Codes


1st Stripe

2nd Stripe

3rd Stripe

4th Stripe (Tolerance)






Table 1. Resistor Color Codes

Figure 3 Parts Placement

You are now ready to prove in the circuit.


Testing the Circuit       

If you are using this kit in conjunction with the TEMP05 module, connect the RELAY05 module up to the TEMP05 via a 16 pin DIP to DIP ribbon cable (not available from Midon Design).  Using the serial interface from your PC to the TEMP05, issue a RLYx ON command (x = 1 to 8).  The corresponding relay and LED should activate.  Do this for all relays to ensure that your soldering and component insertion was OK.  Trouble-shoot as required.

Ordering a Kit       

This project is no longer available for ordering.

Parts List and Suggested Sources


Table 4. Included Parts List



Part Number


JameCo Part #

Other  Source  Part #




LED, Red



1N4148 Diode, silicon 36038  



6-Term Terminal Strip, 6 position 3 x 152346 or 2x 152354  



Relay DPDT 12V Relay, DPDT   Tyco P&B RTE24012F
8 Q1-Q8 2N2222A Transistor, NPN 38236  


R1-R8 1K Resistor, 1K 1/4W 29663  



10KDIP DIP Resistor pack, 16 pin 108628  
1 U1 16PIN DIP Socket, 16 Pin 37372  

Last Updated October 5, 2020

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Greer, SC  29652 USA

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