There is a lot of noise in a Land Cruiser. And when driving on bad roads or off-road, it is very noisy. When you also hear a little badly like I do, it becomes a problem to talk to the sidekick, or hear what is being said over the radio. This gave birth to the idea of an intercom system. It consists of two headsets with microphones, of the same type that gamers use, and an amplifier box with inputs for CB and VHF radio, mobile phone and music / radio.
If you have some electronics experience you can build the box based on this description.
It works so that we can talk to each other via the headsets, listen to music or talk to other cars using a radio or mobile phone. The signals can be mixed together so we can, for example, listen to music and talk together at the same time, or use the headsets as hands-free for mobile and VHF / CB radios.
The amplifier box was designed with as few switches and controls as possible to make it easy to use, and with as few components as possible to make it easy to build. All signal mixers are of the simplest kind. Total price of components for the box is less than 50 USD.
Such boxes can also be purchased as they are used in offroad racing contexts. See e.g. https://www.sampsonracing.com/off-road-intercom-systems-s/1976.htm
Below is a circuit diagram for the design. The diagram is not complete as a construction like this requires you to have some knowledge of electronics. But it is only standard solutions like op-amp bias and decoupling capacitors etc that is not shown.
Read about 2-Din units in Land Cruisers here: 2-Din player with navigation and back up camera
Design
The amplifier is made up of 2 op-amps, 4 headphone amplifiers and two Adafruit electret microphone amplifiers. See https://www.Adafruit.com/product/1713 On the Adafruit card, the original microphone is removed and microphones from the headsets are connected instead.
Everything is soldered on a prototype circuit board that fits in an 12 x 9.5 cm silumin box. It got a little cramped on the card, and if I had to make another, I would use a bigger card. All connections except headset connections are laid outside the card to save space. They consist of standard 3.5mm jack cable extensions that are cut off and soldered to the card, leaving the female connector outside the box.
There are stereo headphone amplifiers, but I could not get it from my supplier and use 2 mono amplifiers per headset instead, a total of 4 pcs. LM386. External components for the amplifiers are not in the diagram, but they are exactly as in the application example from the supplier.
To avoid crossover between the channels, all mono sources, such as CB radios, must have a high impedance buffer in the form of an op-amp to each stereo channel. This is the dual op-amp IC1 -LM1458N. This has a gain of one and is connected as a standard inverting amplifier. The non-inverting input bias circuit is not drawn on the diagram.
Mixing signals from different sources is made as a summation amplifier. There is a mute switch for the music and mobile inputs so you can disconnect these when talking on the radio. Both microphones are connected in parallel to the amplifier. It works just fine, but means that you do not get individual adjustment of the volume on the microphones. The microphones are also connected directly to microphone inputs on mobile and VHF. But NB! a capacitor must be in series with the microphone to get DC separation. The electrets need a bias voltage, and this comes from the Adafruit microphone amplifier. So only AC connection is needed when the microphones first have a bias.
The mic input to the radio do not work properly. The sound is very weak. As seen on the diagram I use a capacitor in series with both mic input wires. I do not think they should share common ground. Both because I am afraid of ground loops, and because I don’t have any diagram of the radio so I don’t know how the mic circuit is designed. Maybe a transformer would be the solution here.
Now I use the radios’ standard microphone. You have to have a push-to-talk switch on these anyway.
The CB / VHF radios are connected to the amplifier from their external speaker outputs via a switch where I can select an external speaker or headset. There is also a switch that chooses whether it is CB or VHF radio to be used. You rarely use both at the same time so no need for separate inputs for these.
A possible add-on could be a microphone for ambient sound with its own volume control. Can be fine when driving in cities and on country roads and want to hear sirens, for example. Or to listen for noises in the car. Anyway, the microphones on the headset pick up ambient sound as well, so this might not be necessary. The Adafruit amplifier has automatic gain control, so this works fine.
I struggled a bit with noise from the alternator when the box was installed in the car due to ground loops. It disappeared when I only grounded the amplifier through cable to 2-Din player. That is, not separate ground for power supply. All cars and connections are different so this must be tested out. In addition, ferrite / notch filters can be connected to all inputs to reduce noise. I had some problems with GSM switching noise from the mobile, but seems like that is OK now.
I’m not an expert in analog electronics design, so inputs on any changes will be welcome.
| IC1 | LM1458N | Dual Op-amp |
| IC2 – 5 | LM386 | Headphone amplifier |
| IC6 | ADAfruit | Electret mic amplifier |
| R1 | 20 k trimpot | Adjust CB/VHF input |
| R2, R3 | 20 k stereo potmeter | Output volume headsets |
| R4 | 20 k potmeter | Volume microphones |
| R5 | 2 k | Mobile need this to detect external mic |
| Capacitors | All 1 microfarad Ceramic | Except when other written |
