Headset project for fire suppression tractors
It is a firefighter’s job to protect the public’s lives and property from wildfires. The firefighter is asked to put his own safety on the line – there is no such thing as making a firefighter’s job safe. However, the South Carolina Forestry Commission, including the communications department (of which I am part), takes every affordable step to improve the safety of our firefighters. Although such steps may not make the job completely safe, it could be said that the safety improvements make the job less dangerous. The basic equipment used in forest-fire suppression in South Carolina is the crawler tractor. Some of the tractors have a blade, some have a fire plow and others have both. (See Photos 1 and 2 at the left.) The focus of this column is on the headset situation used by firefighters operating crawler tractors.
In state government, it is a sad fact of life that sometimes budget constraints are a limiting factor on what can be purchased for our communications needs, even when safety might be adversely affected. We would love to have everything plug and play right out of the box. However, plug and play often gives way to hope and pray that we get something. In short, we learn to make do with less and improvise where possible and when necessary. It is often said that “necessity is the mother of invention.” This is never more true than in public safety communications.
The request At a staff meeting, a forestry supervisor asked me to find out the cost and all the particulars about supplying headsets with boom microphones for his firefighters. A few years before that I had already experimented with a headset and an interface to connect it to the Maxtrac 300 radios that are installed in our fire-suppression crawler tractors. (See Photo 3 on page 20.) Other job requirements forced this project onto the “back burner.” So when the forestry supervisor came to me with this request, I was already well on the way to building a crude prototype that could be offered for trial and demonstration purposes.
The beginning Figure 1 on page 20 shows how the idea was implemented. Here, the interface to the radio consisted of a small preamplifier kit that was designed around the LM386 integrated circuit. The output of the preamplifier was fed through an audio impedance-matching transformer (RadioShack part # 273-1380). This transformer has an 8V primary and a 1,000V center-tapped secondary winding. The center tap of the secondary was used to match 8V to 500V. In kit form, the preamplifier cost was $4.06 from Marlin P. Jones Associates.
Photo 4 on page 22 shows the preamplifier and transformer mounted inside the Maxtrac control head. The small internal loudspeaker was removed to make room for the preamplifier and transformer. The transformer was secured with silicone rubber. Removal of the small internal speaker gave plenty of room to mount the preamplifier and transformer. The internal connections are shown by the colored lines on the schematic in Figure 1 on page 20. As shown in Figure 1, the microphone audio is fed through potentiometer R2 to the audio preamplifier input. A second input to the preamplifier, through resistor R1, feeds the receiver audio from the wiper of the volume control to the preamplifier input. The microphone audio is fed through the preamplifier to provide sidetone to the operator. The sidetone is continuous, whether the operator is transmitting or not. This helps bring in ambient sounds to the operator so that he isn’t impaired from hearing outside sounds, although it is at a much reduced level. In the transmit mode, the sidetone allows the operator to hear himself so that better control of the voice is achieved. It is hard to appreciate this unless you have ever tried to talk without being able to hear your own voice.
The diagram in Figure 2 above shows the wiring inside the interface box that provides connectivity for the headset, local PTT and remote PTT buttons. Photo 5 on page 24 shows the interface box mounted to the side of the radio box that houses the Maxtrac radio. P11 is the eight-pin RJ-45 plug that plugs into the Maxtrac microphone jack, J11. The microphone setup here was designed for a David Clark model H-3440 noise-canceling electret microphone. Bias is supplied from the radio to operate the electret microphone. The remote PTT jack allows a small, remotely located PTT switch to be used to key the transmitter. Photos 6 and 7 on page 24 show the remote-control PTT box. If the remote control PTT box is not desired or not needed, the local PTT button mounted on the interface box can be used to key the transmitter.
Too many requests Initially, the request was to supply the headset capability for about 10 of our fire suppression tractors. We could perform the necessary radio control head modifications and the installations for this many units within a reasonably short period of time. However, as word got around, and the prototype was demonstrated, the request list grew to a level that was beyond our ability to satisfy by doing all the work ourselves.
Why not use off-the-shelf equipment that was already available from some companies that are in the headset business? Two reasons: cost and design. Interface boxes are available from some companies to connect their headsets to many different radio models. However, the interface box was between the headset and the radio microphone jack and had to be worn on the belt or some other such location. In addition, the headset along with the proper interface box was costly. So, what next?
The search for plug and play Now it was back to the search for the perfect headset for our needs at a price that we could afford. About this time I became aware of a company called Peltor. A call to its toll-free number put me in touch with a man who seemed interested in our project and willing to custom-design a headset just for our needs – and at a price that even we might afford. Shortly thereafter, the prototype headset was delivered and the trial period began. See Photo 8 at the right.
The headset did not need an interface box to connect directly to the Maxtrac microphone jack. All of the electronics are contained in one of the ear cups. First, the headset didn’t have enough volume with the tractor running, and the radio wouldn’t scan with the headset connected. Referring back to Figure 2, you will note that the hook switch point is grounded. This allows the radio to scan. If a call is received on one of the scanned channels, there is enough delay to allow the operator to key the transmitter and to talk back to the calling party.
The scan problem was a quick and easy fix. Simply connect the proper wire to ground. The other fix was a little harder to achieve. After trying out the headset, I called the engineering department at Peltor and advised them of the problem. They were a little puzzled at the lack of sufficient volume from the headset but offered several suggestions to increase the volume level. All were implemented with only marginal improvement in the volume level. When a tractor wasn’t available for testing, I plugged the headset into the Maxtrac in my service van. Wow! The volume level was overwhelming from this radio.
So, what’s the difference? A call back to the Peltor engineering department with this information soon led to the discovery. The radio in my van was a different-model Maxtrac from the one in the tractors. It was discovered that the audio feedpoint for the headset (labeled “handset audio” in Figure 1) had a 10k resistor in the line on the tractor radios, and this was killing the audio level at the input to the headset preamplifier. The resistor in the other Maxtrac model was a 560V resistor – quite a difference. With this information, Peltor changed the design a bit and sent another headset for us to try. This headset proved to have sufficient receiver audio.
The field trial The next stage of the process involved placing the headsets with various firefighters stationed around our region for on-the-job testing. The response from the field was overwhelmingly positive. The question became “When do I get one?” The headset was made to order and worked exactly as needed and expected. The price was fair – but not free.
The only thing more impressive than the headset was the willingness of the people at the Peltor company to work with us and find the best design to fill our particular needs – all this with no guarantee of a major purchase. In this age, when many company representatives won’t even bother to return your phone call, it is refreshing to find a company with employees who seem dedicated to the idea of customer service.
Did we buy the headset? Well, not yet – the old budget thing still gets in the way. However, the Peltor headset has been placed on the high-priority list by the firefighters in our region. Having used the headset for awhile, most of them have come to realize just how badly they need them. The headsets provide a good measure of hearing protection from the loud noise of the tractor and allow the firefighter to easily hear and understand radio transmissions.
For a comparison between the regular hand or palm microphone and the noise-canceling Peltor headset microphone, check the electronic version of this article on the MRT Web site (www.mrtmag.com) in October. We created special sound files for this article. The WAV files demonstrate the difference that the noise-canceling microphone makes with the crawler tractor running. And, while you are on the Internet, stop by the South Carolina Forestry Commission’s Web page at www.state.sc.us/forest/index.htm .
Until next time – stay tuned!