RCA Jack to RJ25 Adaptor

The sciBot project uses RCA plugs and jacks as the standard connector for sensors, actuators, and accessories. The mBot has four RJ25 jacks, with the jack number written on the top of the mBot. Unfortunately, the mBot does not come with RCA jacks, so we must make an adaptor that connects RCA jacks to a RJ25 plug that can connect to the mBot.

Jack Colour
Shield
Connection
Centre Pin Connection (depends on port)
Port 1 Port 2 Port 3 Port 4
Red Ground +5V +5V +5V +5V
White Ground 11 9 A2 A0
White (striped) Ground 12 10 A3 A1

On our adaptor, every red jack has a centre pin with +5V, and an outer shield that is connected to ground.

Every white jack has an outer shield that is connected to ground, and a centre pin that is connected to an Arduino analog or digital pin.

There is a piece of adhesive heat shrink around one of the white RCA jacks to indicate that it is connected to the secondary input (A1, A3, 10, or 12). The other white RCA jack is connected to the primary input (A0, A2, 9, or 11).

 The RJ25 jack (registered jack 25) has 6 conductors, and was designed to be used for phones that have 3 landlines.

 

 

Materials

Image Quantity Description Manufacturer Vendor
½
7′ RJ25 plug to RJ25 plug cable Assmann WSW Components AT-S-26-6/6/B-7/R DigiKey A2662R-07-ND
2 x ½
6′ RCA stereo extension cable
Chateau 706-72 Dollar Tree
1 3/8″ (9.5 mm) 3:1 adhesive heat shrink, cut to 80 mm Qualtek Q5-3X-3/8-01-QB48IN-25 DigiKey Q53X038B-ND
1 1/2″ (9.5 mm) 3:1 adhesive heat shrink, cut to 10 mm Qualtek Q5-4X-1/2-01-QB48IN-5 DigiKey Q661-ND
4 1/8″ (3.2 mm) 3:1 heat shrink, cut to 10 mm Qualtek Q2-F3X-1/8-01-QB48IN-25 DigiKey Q2F3X018B-ND

 

 

Instructions

 

Step 1

Cut the RJ25 cable at least 10 cm from each end. In the picture, the cable has been cut about 20 cm from each end.

The cord in the middle can be saved for another project, but it is not very easy to work with.

Step 2

Put the big piece of adhesive heat shrink on the RJ25 cable.

Step 3

Cut about 20 mm of the outer insulation from the cable.

Step 4

Orient the cable with the clip up, facing away from you. Cut off the 2 left-most wires.
Note the the colour of the wires may be different from one cable to another.

Step 5

Strip about 2 mm of insulation from each of the 4 wires.

Step 6

Tin the exposed copper wires by heating with a soldering iron and adding a bit of solder.

Step 7

Cut both RCA extension cables about 15 cm from the jacks.

Save the plugs for connecting other sensors and actuators to the sciBot.

Step 8

Pull apart the cables, and strip about 20 mm of insulation from each cut end.

Step 9

Strip about 3 mm of insulation from the two white wires. Twist together the bare wires.

The bare wires are all connected to ground, and each white wire will be connected to an input/output pin on the Arduino.

Step 10

Strip about 7 mm of insulation from the two red wires. Twist together the bare wires, and twist together the red wires.

The bare wires are all connected to ground, and the red wires are all connected to +5V.

Step 11

Twist together all 4 bare wires.

They are all connected to ground.

Step 12

Tin the wires.

Step 13

Put the short pieces of heat shrink on the RJ25 cable. With the clip up on the RJ25 jack, and the jack pointing upwards, solder the 4 bare wires to the left-most wire. Solder the 2 red wires to the second-left-most wire. Solder one of the white wires to the right-most wire, and the other white wire to the second-right-most wire.

Step 14

Put the small heat shrinks over the solder joints and heat with a hot air gun.

Step 15

Put the large heat shrink over the entire joint, and heat.

Step 16

Put the small piece of adhesive heat shrink over the white RCA input that is connected to the secondary input (A1, A3, 10, or 12) so that it is easy to tell which input is which. The example code ex_AnalogRead2 can be used to identify the secondary input.