A Wireless PTT SwitchFor Mobile Operations
By Geoff Haines, N1GY
In recent years the virtues of “hands free” operation of things like cell phones have been debated at length. Several states have enacted laws requiring “hands free” operation. Unfortunately, although most of these laws specifically exempt amateur radio, the holding of a standard hand microphone while driving may result in a discussion of the law with a local or state law enforcement officer.
When I developed an adapter to permit the use of so-called “universal” type cell phone headsets (or earsets, if you like) I saw an opportunity to adapt this idea to my mobile set up. I wanted to have both the hand mic and the ear set available for use, so a mic switcher was the core of the project. In order to provide a PTT for the ear set some way would have to be found to place the PTT switch on the steering wheel. Anywhere else and the concept of using an ear set does nothing for the idea of “hands free”. The usual way to accomplish this is to place a wired PTT switch on a spoke of the steering wheel with a coil cord not unlike a telephone handset cord running around and down the steering column where it connects to the radio. Aircraft, motorcycles and racing cars have used this technique for years. The biggest problem with this for street driven cars is very basic: The aircraft, motorcycles and racecars have a very limited range of motion of the steering mechanism. Usually no more than 180 degrees lock to lock. That is from full left to full right turns the steering wheel or handlebars or yoke in the case of aircraft only makes about one half rotation of the steering shaft. In this situation a coiled cord from the PTT switch has little opportunity to get fouled in the steering gear. The average street driven vehicle on the other hand has as much as 3 to 3 ½ turns lock to lock. It also has turn signal and shift levers, cruise controls and air bags along with the steering function. These all create hazards for the PTT cord and thus for the driver.I wanted to devise a better way to operate the PTT circuit of the radio, one that would not involve wires at all. A number of circuits and devices were researched, all seemingly expensive or unreliable.
I happened to be discussing the problem with a fellow amateur one day and he suggested a simple solution. A garage door remote and receiver! Bob, KI4HXT told me that he worked in that field and that a single channel transmitter and receiver could be had for the asking. No better offer than that, for sure. We got together and Bob showed me how they worked. A relay on the receiver board will stay closed as long as the push button on the remote is pushed. The relay releases as soon as the remotes button is released. Sure sounds like a PTT switch to me. If the antenna is removed from the receiver, the range of the remote is decreased to about 5 or 10 feet. Encoding of the transmitter and the receiver keeps everyone else’s garage door remote from keying my radio.Bob explained that some models of receiver run on 24 volts while others run on 12 volts and some can do both. The remotes are usually powered by a small 12 volt battery about half the size of a AAA cell or a 9 volt battery just like the one in many small consumer grade AM/FM receivers. Some of the 9 volt units may operate on the 12 volt cell but this would require a different battery holder. This could be done, however, to repackage the transmitter into a better size and/or shape for mounting on the steering wheel until you find a key ring type. As long as the remote and the receiver are compatible, meaning the receiver can read the remotes coded transmission all will be well.
These devices operate at around 300 MHz and are under Part 15 of the FCC rules. The encoding and decoding of the signal from the remote to the receiver is done by the manual setting of a ten position DIP switch in each unit to the same code. Other units use different coding methods which are more expensive. The DIP switch coding should be just fine for this application. The units I used for this project are made by Linear Corporation under the trade name Multicode. They are a single channel remote and receiver system. The specific models are Multicode 3070 for the transmitter remote and Multicode 1099-50 for the receiver. Many, many thanks to Bob, KI4HXT for all his help and advice with this project.
I took home the used units Bob donated to the cause and wired the receiver up for my project. There are four wires exiting from the receiver case which is about 6” x 4” x 1 ½”. Two are grey; they are the leads from the relay. A 1/8” mono plug was wired to these to fit the PTT jack on my mic controller/selector. Closing the relay closes the PTT circuit for the transceiver in my SUV. The other pair of wires are our old buddies red and black and they get wired to a source of 13.8 volt DC. Since my Icom 706MkIIG is fed from a master on/off switch in the power line from the battery of the car, feeding the PTT receiver from that same source means that when the radio is turned off, so is the PTT receiver thus no current draw while parked and unattended. The battery for the remote only sees current draw when the button is pushed and they are readily available at most battery stores.
The remotes come in several sizes. Some are meant to be mounted on the visor of the car; others are designed to hang on a key ring. This latter type (the Multicode 3070) is the best for steering wheel mounting as it is only about 2” x 1 ½” and only about 5/8” thick. It even comes with a Velcro mounting system so you can easily strap it to a spoke of the steering wheel with hook and loop ribbon. Most of these remotes cost around $25 although I have seen them offered on-line for as little as $14. The matching receivers usually cost around $35 to $40 and again online prices will vary widely.Okay, so just what have we accomplished with our little gadget. Well, first we have a reliable way to control the PTT of the radio without taking our hands off the wheel. We also do not have a coiled cord wrapped around the steering column, sometimes getting caught on the turn signal lever or the ignition keys. We can operate safely and still have the normal finger or thumb operated push to talk type operation. No VOX going wild every time a truck drives by or when your window is open and the wind noise picks up. Combined with a cell phone type ear set the appearance is of the driver following the “hands free” principle.Now, how do we connect this to the amateur transceiver? As I mentioned before, I wanted to have the option of using the regular hand mic or the cell phone ear set. To do this I needed a switching mechanism. As has been written about by others in the pages of QST, I chose to use an old A-B Data switch from the days before computers had USB hubs. These are available at computer shows and hamfests for mere fractions of what they cost new and alternatively are still produced by some aftermarket computer gear manufacturers for really not a lot of cost. As long as the original device has enough pathways to satisfy the needs of your radio to mic connection and for switching the receive audio from the external speaker to the ear set you are all set. Then it becomes a matter of substituting microphone connectors for DB-15s or DB-25s and the like. I have included a circuit diagram that I use for this type of device. They work very well. I have installed similar mic switchers on five of my radios from Alinco DR-150s to Icom 706s and even a Yaesu 7800.The circuitry to adapt the cell phone ear set to the transceiver desired is very simple. Usually just one resistor, the value of which matches the impedance of the stock microphone and a polar capacitor to keep the DC off the mic + line to the radio. Some of the Yaesu rigs like the 7800 and 8800 need a second resistor in the PTT line. Normally something around 27K Ohms works well. The 706 and the DR-150 just ground the PTT line for transmit.
Some Yaesu rigs of late want to see that resistance instead of just ground.These few components can be installed inside the mic selector at the 2.5 mm stereo jack for the cellular ear set. The PTT line for the ear set is run to a 1/8” mono jack where the relay of the PTT receiver plugs in. Remember to connect an external speaker to the selector as well; otherwise your radio will be very quiet when using the standard hand mic. The mic selector circuit diagram here shows how the receive audio is sent to either the external speaker or the phone of the ear set.The selector shown in the photographs was repackaged into a Radio Shack enclosure since the original enclosure was too big for the site in the car. If you have more space you can use whichever you prefer.Speaking of Radio Shack; I should mention that I purchased the ear set that I use in the car from them. List price was less than $7.00 and it has adjustable volume for the earpiece AND adjustable sensitivity for the mic side of the device. Using it with my Icom 706MkIIG, I had to turn the mic sensitivity down to minimum to keep it from overdriving the radio. Nice little gadget that.I am very happy with the results of this project. I have the choice of the stock hand mic or the cellular ear set with the PTT for the ear set right on the steering wheel. I do not have to take my hand off the steering wheel to answer another ham on the frequency and there are no wires to get tangled up on the column.
The only major problem I encountered was some interference when the radio was set to any frequency from 147 MHz to 148 MHz. This was cured by putting two inexpensive ferrite chokes on the power wires to the remote PTT receiver. Apparently the feeding of power to both the IC-706 and the remote receiver caused interference to the remote receiver. With the chokes in place at both ends of the power lines to it the interference disappeared completely. All of the other frequencies and bands appear to have no problem.
N1GY- The simple Approach to Ham Radio