The DRA-818 UHF and VHF transceivers

The DRA-818 is a transceiver in a roughly postage stamp IC. The ICs they use internally are similar to those used in other import radios. They do not have the ancillary circuits such as audio amplifiers or battery charge handling. There are two distinct flavors of these modules, a 134-174mhz version and a 400-470mhz version. Supply voltage is rated at 3.3 to 4.5V however I’ve used these on a 5V supply with no problem or salvaged 18650 laptop batteries.

There are a high power of .5/1W RF output and 8 selectable audio levels. Controlling is done with standard 9600 8/N/1 serial protocol. Keep in mind a voltage level converter is required to go from RS-232 to levels acceptable to the IC. I’ve used a MAX232 variant with no issues. Although there are 18 pins, there are only 12 used. These are:

-Squelch detection (inverted logic output)

-Squelch detection (inverted logic output)

-speaker output

-PTT (TX low)

-Sleep mode (active low)

-RF power selection (low .5W, floated 1W)

-power (3.3-4.5V)

-2 ground pins

-Antenna (50 ohm nominal output)

-RXD (serial input TTL levels)

-TXD (serial input TTL levels)

-mic input

-mic input


In my experience with this IC it’s very particular with newlines when formatting input data. I spent a decent bit of time researching, a few blog posts by others and trying a few methods to get it working. I ended up settling on Python for dealing with this. Code will be in the next post.

6LU8, 6LR8 triode/beam pentode combination

The 6LU8 and 6LR8 are essentially the same tube, the difference being a Novar (giant 9 pin) vs a Compactron (12 pin) pinout. As designed use of this tube was for a combination vertical oscillator and amplifier role in televisions. Physically in the picture you can see the separator and the 2 sections. On the left is the beefy pentode on the right is the Triode.

This tube was produced in quantities that make it good for experimentation. A way that you can possibly save cost on purchase is to buy one of the tubes designed for stringed filament service in 16V or 21V. That said even the 6V filament variants currently run well under $10 on Ebay.

The design maximum plate dissapation is 14 watts which is similar to the 6V6. Plans for “one tube” transmitters would likely be a good starting point for a design using these tubes. The Triode would end up being used for a crystal oscillator or buffer amplifier if one is in to more modern frequency control. There are smaller tubes based on a similar premise but they’re limited to tubes like the 6AF5 and variants.



The LM393 is the smaller sibling of the LM339. Whereas an LM339 has four comparators in it, the LM393 has two. As such there’s a nearly limitless amount of things that can be done with this IC (essentially any project where a section of an LM339 is required.)

This IC take a pair of analog voltage levels and effectively converts them into a digital signal (high or low.) When the negative input is higher voltage than the positive input, the comparator goes low. When the positive input voltage is higher, it swings high. Besides being used in comparator applications, another set of applications are digital voltage level converters and output buffers for micro controllers with lower current sink capabilities. Its input voltage limit is 36V rail to rail with negative rail supported. This allows a wide variety of applications including automotive voltages.

We hope you enjoyed this blurb on the LM393, catch you next week.


Active of the Week

This will be a spot where a random active component will be posted. Usually it’s things that were stumbled on in every day life and have some relationship to Radio or some outlier. A few interesting things will be said about the component, why it’s valuable and what its potential interests would be in radio. This can be anything from a discreet component such as a transistor or tube to an integrated circuit of some kind.

To that end, I’m happy to announce that our first active component will be the 6J6 vacuum tube. This little guy is a dual triode with a very unique construction. If you notice the cathode (grey, in the middle) is shared between two units. The copper colored rods hold the wires for the grids and the blade outer pieces of metal are the plates which are totally separated from each other. Some other tubes have shared cathodes on paper, but are actually two individual cathodes which are tied to a common point.

The realm of a tube like this is simple in most cases. It’s designed for a mixer circuit and anything else that requires balance with a measure of precision. With a conventional dual triode such as a 12AT7 matching becomes an issue as does how evenly emissions changes. With a tube like this that becomes less of a concern. Although different in function (they are beam deflection tubes vs a conventional Triode 7360s and 6AH8s are similar in principle that two plates receive emissions from a single cathode.

I hope you learned something from this. Stop by next week, it is likely I will have an unorthodox transistor on the bench to look at. If not, who knows what it will be.