Another SWR meter : the "Blinky SWR"

First some history ...

Long time ago (in 2003!) I built a circuit that showed SWR with  four multicolour LED's. It was designed in the NJQRP Club for their Rainbow Tuner, by Joe N2CX (SK).

Although the kit is no longer available, the info is stil found here.
And in case you're interested, I still have the Eagle files for the circuit and a PCB I designed for it, just send me an e-mail on my QRZ.com address. Note : my version does not include the tuner part.

The nice thing about it was that it is fully automatic : the comparator is fed with incident and reflected power from a resistive bridge, so it needs no adjustment for "full scale" forward power. Also the battery power is automatically switched on and off.

But it needs a battery ...

More recently I came across the TinySWR, another circuit that shows SWR with LED's, and this one needs NO BATTERY, nice !

It's a design by Martin, DK3IT, based on work by others. 

The circuit is completely passive, and energy to lit the LED's is taken from the RF you put into the SWR bridge. Info about the TinySWR is here , and you can buy a kit with all components here.

Here is how I built a TinySWR into an EFHW tuner.

Still a disadvantage : you have to wind a tiny toroid transfomer for the bridge, and this scares some people off.

The Blinky SWR

Recently, I saw a link on the SOTA reflector to another solution, the Blinky SWR.
Aha,  now we're getting somewhere ... no battery and no transformer !
The design is quite sophisticated, it uses an AtTiny13A microprocessor, which gets it power from rectified RF, and uses the same resistive bridge like the Rainbow Tuner above.

SWR is shown with 6 LED's, and by combining intensity of adjacent LED's, 21 levels can be shown.

Another clever trick is that the Blinky shows the transmit power (max. 10W) after you stop transmitting, with the remainder of the energy stored in the two big capacitors. 

Hmm ... there is still something that may scare some people off: you have to solder a tiny 8-pin SMD chip and 6 even tinier SMD LED's. Well, that didn't scare me off, and I ordered the kit here.

It arrived in a regular mailing envelope in less than a week, wrapped in several layers of paper and bubble plastic. It took me less than an hour to assemble. To make sure the PCB didn't move while soldering, I fixed it with some Scotch tape onto a plastic box. Here you can see I already soldered the CPU chip and three of the LED's.

And here a detail with all components soldered.

I didn't want to put the PCB into one of my QRP transceivers, instead I found a Pomona box with two female BNC connectors. This way, I can use the Blinky in any situation.

Now, to reduce the number of jumper cables, I replaced one of the connectors with a male BNC, so I can put the Blinky SWR directly on the output of a transceiver. Also there can't be any confusion of where is the input or output.

Using some stiff wire, I mounted the PCB inside the box, hanging between the two connectors.
I made a rectangular slot in the lid, and fixed a piece of plexiglass at the inside (cutout from a CD box), as a viewing "window". See top of this post for the final result.

To check if the Blinky really does what it claims, I made a few test resistors on two BNC connectors.

one was 150 Ohm, the other 470 Ohm, which should roughly give an SWR of 3 and 5 respectively.

The resistors I used may be a little inductive, but all they have to do is give a "bad" SWR for comparison. I also used a purely resistive 50 Ohm dummy load.

Checking and comparing with an "good old analog" SWR meter, I can confirm that the SWR shown is quite accurate (within the normal ham tolerances of course, hi), and so is the power indication.

Beware though, the power indication is only correct when transmitting into a 50 Ohm load (dummy load or well tuned antenna).

I did also a test with my Z-Match tuner (see this post), and must say that the Blinky SWR is even more sensitive to detect small differences in SWR than the single LED indicator in that tuner.

Now, if only I could remember where I left the little plastic bag with the four screws to fix the lid ... HI (see top picture).