An Unconventional Assembly: QRP-Labs Ultimate Relay Switched LPF Kit

The QRP-Labs Ultimate Relay Switched LPF Kit was developed by Hans Summers, G0UPL, as an extension to the Ultimate 3 and Ultimate 3S transceivers. It allows switching between six different Low Pass Filters (LPFs) and so operating the transceiver on six different bands instead of one.

I’ve used these relay-switched filter boards in several projects now and I’ve been surprised at how easily they can be adapted to uses beyond their design. The build below shows an unconventional use for the kit – switching multiple filter arrays from a single MCU pin. This is a part of a 10-band POTA rig I’m building around the HackRF One.

For details on the kit and more conventional uses of it check out QRP-Labs.com.

Pros

• Low power consumption, 10W+ switching.
• Easy to build and integrate into projects.
• Swappable filter modules.
• Easily adaptable from an LPF switching array to a BPF (Band-Pass Filter) array and other applications.

Cons

• Closed source hardware.
• Uncertain inductances (performance may degrade at frequencies higher than ~30Mhz).
• No input terminal, output terminal only usable in LPF configuration.
• Missing thermal reliefs on some ground pads (makes soldering slightly more difficult).

Safety Matters!

Soldering irons are hot. Everything they touch gets hot.

Have a fire extinguisher nearby!

Solder splatters. Your eyes are not easily replaceable.

Wear your PPE!

Tools and Materials

1. Soldering Station
2. Multimeter
3. Helping Hands
4. Heat Gun (not pictured)
5. Lead-Free Solder
6. Solder Wick
7. Tip Tinner
8. Flux Paste
9. Solder Sucker
10. Mini Pliers & Cutters
11. Jewelers Loupe
12. Ribbon Cable Jumpers

Parts Included

1. 6x G6S-2 5v DPDT relays
2. 6x 1N4148 diodes
3. 10x 4 pin header sockets
4. 1x 2×5 pin header socket
5. 1x 2×5 pin tall plug
6. 4x nylon spacers
7. 1x PCB (not pictured)

One relay and diode, the 2×5 pin header and socket, and the nylon spacers will not be used in my build today. The extra relay and diode were included with the kit to modify the Ultimate 3 transceiver but I’m not building this kit for that purpose. The spacers, header, and plug are intended to attach this board to the Ultimate 3 but don’t fit in my project.

Assembly

Install the diodes first as they the smallest components and will be very difficult to install later.

Align the black band on the symbol with the PCB’s silkscreen. I’m top-soldering to secure the parts before I flip the board over to finish the solder joint.

Finish the diodes by trimming their leads as close to the board as possible. You need these as jumpers to configure the board!

Fold the off-cuts over your pliers to form a tight bend.

Solder the jumpers in place following the kit’s instructions. I’m configuring the board as if it were switching BPFs. Each relay channel will be independent rather than always routing through relay one.

I’m top-soldering these components too, before flipping the board and finishing the joints from the bottom. It may not be the most professional technique, but I don’t feel like chasing little bits of wire that fall off my work bench.

I’ve made my first mistake on this build! At least the first one I’ve noticed.

On the right side of the board three jumpers are very close together and I’ve created a solder bridge. Looking closely the ends of the jumpers are bent out and touching each other too.

It’s time for some flux and the solder sucker. Flux will improve heat transfer and increase the surface tension of melted solder.

Don’t be stingy with the flux! You can always clean the board later.

Once the excess solder was removed I also trimmed the ends of the jumpers a bit to keep them apart.

The solder bridge may be gone but a close inspection shows a lot of solder residue.

A bit more flux and a brief pass around each pin with the soldering iron will pick up that residue.

With the small parts installed the next step is relays.

I use a bit of flux to hold the relay in place and improve flow from the bottom of the board through the joints. Ideally through hole joints have solder all the way through the board.

It’s a good idea to solder a single pin to secure the relay and check for any misalignment before soldering the rest of the pins.

For the second relay flux wasn’t enough to hold it in place but it’s safe to hold the relay with one hand and solder with the other. Just put a bit of solder on the tip of your iron for the first pin.

Yes, I know transferring solder from the iron to the pin isn’t good technique. It’s one pin to hold the part and I’ll improve the joint after the rest of the pins are soldered.

Here’s another mistake. The relay pins connected to the ground plane didn’t form a solid joint. See the bare copper around the solder joint?

This joint needs more heat. The designer didn’t include thermal reliefs on these pins so the ground plane is pulling heat away; solder only flows to hot metal.

Turning up the heat on my iron and adding some flux might have worked but I didn’t want to struggle through all 10 of these pins.

Enter the heat gun! On a low setting for a few seconds the heat gun warms up a large area of the board enough that the soldering iron can get that joint hot enough to flow freely.

Inspection with my loupe again shows a solid joint.

Always inspect your joints! Your eyes aren’t as good as you think they are and even experts make mistakes. If you’re reading my blog you’re probably not an expert anyway (I’m not!)

Entering the home stretch… module headers are next.

I tacked one pin on each header first to secure them and check for alignment while it’s still easy to fix.

Surprise! Surprise! A mistake!

One of my headers is misaligned. Since I’ve only soldered one pin it’s easy to fix the alignment by heating up the joint and nudging the header into place.

If you’ve soldered more than one pin you’re going to need to remove some solder or use the heat gun to heat up all the joints (good luck not melting the plastic while you do that, you might need to replace the header).

Once all the headers are aligned the rest of the pins can be soldered. Be generous with the flux here, solder doesn’t like to flow over oxidized pins without a little help.

Next the board needs control lines and power. I decided not to use the supplied 2×5 pin header and solder wire directly to the board.

This type of header isn’t the most secure choice for wiring. I expect this rig to be bumped around a lot and I really don’t want to be troubleshooting disconnected wires in a park or on the side of the road.

That’s it! In a future post I’ll show how this component fits into the larger project.

Until then, see you on the bands!

73, AI6YM