Text: Bob Fisher
I have an MFJ-434 voice keyer, which has saved me a lot of effort over the years. It has always been an important accessory when I call the CQ during amateur radio competitions and when I use low-power (QRP) transceivers.
One of my favorite new pastimes is operating QRP stations away from the noisy radio jams of the city. Most of the time, it's important to pack only the most basic necessities for these mini DX expeditions. I've never used an MFJ keyer, mostly because of its size. An "accessory" measuring 6-1/2 x 7 x 2-1/2 inches is filled with backpacks. It also weighs 1.5 pounds. On the other hand, I'm sure that not having keying won't cause me to have fewer radio connections. I always yell at my throat to complete A CQ mission before the battery runs out.
Last winter, I was packing up a QRP "Go Box" for a trip to Florida. Since the planned stay was longer than my normal field trip, I decided to take my MFJ keyer for my first outing. How different are keyers! In less than three months, I used a 10-watt transmitter and dipole antenna to connect stations in 31 countries and five continents!
A large number of communication links require careful study of keyers in order to conduct field trips. The MFJ-434 has 11 buttons to press, three potentiometers to turn, and two LEDs to show that keyers are following my instructions. Can I use fewer amenities and reduce my size, weight, and power requirements to a more backpack-friendly level? It's worth investigating.
The MFJ keyer stores five messages. For field operations, I can tolerate a CQ. I found a 20-second record playback module on eBay for $2.59.
I also found a recurrence timer (variable on and variable off delay) on eBay for $2.38. I can use it to "turn on" a message, then "turn it off" for a predetermined amount of time before turning it on again.
Each time I play my CQ message, I can add a relay to control the transmitter's PTT circuitry. The receiver automatically listens for any replies between messages. (This might really work!) ）
Using all the major components provided by eBay, I sketched and combined them in Figure 1 below.
The circuit is simple and straightforward. Although I designed it to work with the Microbitx (μBITX) high-frequency transceiver, it can easily adapt to any transceiver that uses a push-to-talk (PTT) microphone. The three-pin wire connects the two ungrounded contacts of the stereo headphone plug to the center pin of the manually operated three-pole, double-throw (3PDT) pushbutton switch.
In the Keyer Off position, the switch connects the original PTT microphone to the microphone input jack of the intercom. In the Keyer On position, the switch connects the output of the record/playback module to the transceiver. The 3PDT switch also controls the circuit's 12 VDC supply.
Both the record/playback module and the repeat cycle timer are 5 VDC devices. Whenever the keyers are turned on, the DC-DC "buck" voltage regulator powers them.
When the push button switch turns on the keyer, the repeating cycle timer is energized and its normally open output contacts are closed within the length of time the delay potentiometer is switched on. At the end of that time, the N.O. contact opens and remains open for the length of time that the timer's disconnect delay potentiometer is set. At the end of the shutdown delay, turn on the delay loop to start again. As soon as the timer is powered on, these cycles continue to repeat.
The normally open output contact of the repeat cycle timer is used to energize K1: a bipolar double-throw relay during the cycle. The relay (K1) simultaneously controls the transceiver's PTT circuitry and "turns on" the playback module. The recorded message is sent to the microphone input of the intercom.
R1 is a 10K ohm potentiometer that sets the transceiver's audio input to around 40-50 mV. I start with R1 zero and increase the microphone input until the transceiver RF output is equal to the microphone output.
Recording information sent to the intercom microphone input can also be heard through the keyer's speakers.
C1 is used to isolate the record/playback DC voltage from the transceiver. The D1 acts as a "buffer" to short-circuit the back EMF voltage spike between the relay coil and ground.
I had a nice used spare case, so I found a 50 μF capacitor, a two-pole double-throw relay, a 3.5mm stereo jack, and a perforated board inside.
Even though I have some 3PDT switches on hand, I still choose to buy a Foot Switch from eBay. It will get a lot of use, and the heavy duty description is attractive to me.
Shortly after the last part arrived, I spotted the individual components together. The mixture shown in Figure 2 looks like a very attractive combination.
Figure 2
The only bad news is that I can't put all these boards into the beautiful 1-1/2 x 2-1/2 x 5-inch project box I plan to put them into. If you want to get a bigger box, you will at least have to go to a long store to find it.
Luckily, growing up on a small farm gave me a lot of hands-on experience with innovative art. At times like these, I remember my father telling me, "You can't run into town every time you need something." Look around and see what we have to get you to work. ”
I found a 2-3/16 x 3 x 3-5/8 inch plastic junction box shown in Our local Lowes store.
Figure 3
It works. Some work is required with the Dremel tool to remove the nail bracket used to secure the box to the wall post. Moreover, it still looks a lot like a wall-mounted electrical box. However, that box allows me to complete projects on my schedule.
Five minutes of Dremel's time knocked out two nail brackets. I drilled a screw hole to mount the board and speakers and sprayed a layer of black paint on the entire junction box. After another hour, the components were fixed. The finished product works well. It doesn't look scary, but I might decide to repackage it at some point in time.
Now, I'm excited to have a single-message CQ keyer that's only 23% and 1/3 the size of the commercial keyer it replaces. The only drawback I found when testing my new keyer was that it consumes 110 mA when calling CQ. This is an unpopular situation. The MFJ-434 consumes only 65 mA on a call. They consume 15 mA between calls.
The extra 45 mA won't be a game breaker for me. When I answer voice, the transmission duty cycle should be significantly less than 50%. Running the keyer for eight hours without any launch time to answer the conversation would only consume 0.5 amp hours of the 12 Ah battery I brought to the site. If my battery runs out of battery, it will be because I do more communication and my vocal cords will make sound anyway.
This keyer can also be used with Voice Operation Relay (VOX) circuitry. This is the solid-state version of the "Beacon CQer" that I built around a cassette tape recorder and 90-second infinite loop tape in 1976.
IsD-1820 recording playback module with speakers
5V DC Infinite Cycle Delay Timer Relay ON/OFF Switch Module
DC-DC buck converter, 5A, 4-38V input, 3.3-24V output
3PDT 9 Pin Guitar Effect Pedal Switch Pedal Box Foot Pedal Metal Switch
DPDT relay, 12 VDC coil, virtual P/N VRS2H-S-DC12V-C (or equivalent)
1N4148 Switching diode 75V, 0.15A
50 μF 50 VDC electrolytic capacitor
10K ohm potentiometer
3.5 mm stereo headphone jack (or appropriate microphone connector)
PVC wall box and blank cover
Screws, nuts and washers for Ace Hardware
source: nutsvolts.com
Here comes the uncle:
Do you have a habit of using voice keyers? Or do you love the thrill of yelling, welcome to share your experience of communication!