Build This 5Hz to 400KHz LED Sweep Signal Generator From Kits

Build this easy sweep signal generator from readily available kits.

If you had a look at my last instructable (Make Professional Looking Front Panels), I might have eluded to what I was working on at the time, which was a signal generator. I wanted a signal generator where I could sweep through frequencies relatively easy (Not just set and forget). As I couldn't find anything cheap, I decided to piece together one myself and using kits as a basis.

The heart of the project is a signal generator kit which is easy to get off Ebay, Amazon etc. It's easy to build and customisable. There are four frequency ranges (5-50Hz, 50-500Hz, 500Hz-20Khz and 20KHz-400KHz), three types of output (Square, Triangle and Sine).

The counter is another kit and counts from 1Hz-75MHz with auto ranging and 4 or 5 digit resolution.

A Couple Of Notes:

1. I didn't design these kits, only built them as part of the project. They're readily available through most online outlets (Ebay etc). That being said, if you have issues with parts, building etc there's no use contacting me about it. Contact the seller you bought it from. I'm happy to try and answer questions in relation to how I've used them in this particular build however.

2. The frequency counter kit, while it says it will count from 1Hz to 75MHz I didn't find that the case. The slower the frequency got, the slower it and larger the error margin. If anyone knows of a better counter kit, I'm happy to hear about it. As it was, this was the best one I could come up with that will read lower frequency values (Sub KHz)

ICL8038 5Hz - 400KHz Frequency Generator kit (Off ebay) about $12-13

1Hz-75KHz Frequency Counter Kit (Off ebay) about $12-13

LED On/Off Switch (you can use any you like)

4 Gang Push switches (usually come as DPDT - this might be a hard one to track down). You could use a rotary switch if you can't find one.

1 DPDT push switch (I had singles of the matching gang switch)

4 Pots (2@5KB, 1@50KB) (I used a 50KB multi-turn precision pot for the frequency adjust)

3 BNC panel mount connectors

DC panel mount connector

1x Large Knob (To suit 50mm pot)

Male/Female PCB standoff connectors and plugs (Various sizes)

Right angle male PCB standoff connector

Brass standoffs (Various sizes)

Instrument case (most expensive part of the project)! about $25

Inkjet white & clear paper

Optional:

1 x 5.5mm DC connector (signal generator board)

1 x 4mm DC connector (meter board)

Because I already have a lot of this stuff, the cost was about $50 (2 kits plus a case), but may be higher if you don't have connectors, stand offs, knobs, switches etc.

Basically it's just a signal generator kit with a frequency counter connected to the output. However, I've added in a few handy switching combinations.

There's 3 BNC connectors:

One for the main output (that's always in circuit unless you switch the measure switch to external), one BNC for int/ext measuring using either the internal meter for an external source and one BNC on the rear panel that is connected to the above (So you can connect either via the front or rear panel).

The int/ext switch is used to switch a signal to the internal meter. If it's in the internal position (in), the signal from the generator goes to the meter and all the BNC connectors. With this config you can connect any external measuring gear (Frequency counter, oscilloscope in parallel with the main signal out). If the switch is in the ext (out) position, it disconnects the main out and both the int/ext & rear panel BNC's are connected to the internal meter. So you can feed in an external signal and use the internal meter to measure it.

The Signal Type switch is a rotary switch that basically switches between Tri/Sine in the first two positions. The opposite switch connects the tri/sine signal to the output. In position three, S1a isn't used and is only switching between the squ & tri/sine outputs to the main output.

Before you go out and spend money on one of these frequency counter kits, they're not all the same. Essential what you want is a kit that measures lower frequencies. A lot of the ready built modules only measure 1MHz and above. There's also some kits out there that look similar, but the code of the main chip isn't correct from the original design. That is why I've chosen this particular kit as it's the only one that even resembled working correctly.

From the sellers site, the specifications are as follows:

• 1Hz-75MHz
• Four or 5 digit resolution depending on frequency being measured (i.e x.KHz, x.xxx MHz, xx.xx MHz)
• Resolution 1Hz (max)
• Input sensitivity <20mV @1Hz-100KHz, 35mV @20MHz, 75mV @50MHz
• Input voltage 7-9V (works on 12V no worries)

Build the counter kit as per the sellers instructions with the following modifications:

• Use PCB connector standoff's for easier plug and connect later
• The on/off switch is optional and you can just link it if you like or install it (you have the switch there so why not)!
• Mount the red variable cap on the underside of the board (In the photo it's mounted as per the recommended build, but I've flipped the board over). I changed it's position and you'll see that in later photos.
• Use a right angle inline connector instead of the straight one supplied to side mount the LED screen. That way it can stick out into the case and not all over your bottom controls!
• C14 is apparently not used (I think it depends on what range of variable cap is supplied and to set the meters accuracy). Personally, I don't think it matters as the variable cap doesn't add a whole lot of calibration even by adding a small amount of extra capacitance at C14.
• The variable cap supplied (red 5-20pf) was garbage and needed replacing. I ended up buying a mixture of different caps (50 or so) of various values as most supplied with kits seem to be garbage.
• R14 is supplied as a 56K resistor. This can change according to different batches of C3355. For this reason, I mounted a couple of pins from an IC socket so the resistor can be readily changed if need be.

Once you've built it, check the functionality against a known signal generator source.

Notes:

While the documentation says this kit will measure 1Hz to 75MHz, in reality I've found (like most kits) it measures better at higher frequencies. This is the reason I've added external BNC sockets to connect more accurate equipment. It also tends to display different results depending on whether the signal is sine/triangle or square. The slower the signal, the slower the measurement time. It gets it in the ball park most of the time from about 500Hz onwards. Again, if someone knows of better kit, please let me know.

From the sellers information, it's specification are as follows

• 5Hz - 400KHz working range
• Duty cycle 2% - 95%
• DC bias adjust -7.5V to 7.5V
• Output Amplitude 0.1V to 11V PP @12V
• Distortion 1%
• Temperature Drift 50ppm/Deg C
• Voltage +12-15V

Again, build the kit as per the sellers instruction with modifications of the following

• Use PCB standoffs for easier connections later. This is for all pots (R1, 4, 6, 5), JP1 (Tri/Sine select), JP2 (Freq range select) and JP3 (main out)
• Once completed, you can temporarily connect pots and jumpers to check if the board is functioning as expected by connecting it to an oscilloscope.

I won't go through the whole process, only what I did different to my other instructable on "Making Professional Looking Front Panels". I've also included the Front Panel Express design file so you can print one the same if you like.

Basically start by tracing your front panel and doing a mock up of how you want it to look. I've included the penciled version I started with. Add dimensions where you can as it will make it much easier when it comes time to input it into front panel express. Towards the end of this Instructable I may add some iterations of the project if I have photos.

Your front panel dimensions will be determined by the project box you use. I got this particular one from Jaycar (it's the larger instrument box). I started with the smaller one's I normally use, but had trouble fitting everything I wanted onto the front panel (with the switches, LED counter, controls etc). So went with the larger box.

Use the software to design the front panel. Then print out two versions: one black & white version onto normal paper for drilling (with hole centres) and one final colour version onto a white label sheet.

Once you have your drilling template, stick it onto the panel, mark your holes and drill the holes and cutouts. Once all done, remove the template and thoroughly clean the surface with a grease and wax remover or spirits. Use a tack cloth to remove any fine dust particles before proceeding to stick the panel label on.

For this particular build, I only used inkjet paper. If you look closely you can see a little bit behind the paper. In this case I'd suggest either purchasing non-see through label stock or, use stick one half of the unused sheet first, then put the printed panel sheet over that. To finish off, place a sheet of clear inkjet film over to protect it all. You can leave some over-hang, cut the corners at 45 Deg and wrap it around the back of the panel as well.

To finish, cut out all of the holes using a sharp craft knife.

Screw all of the pots, BNC connectors, rotary and power switch onto the front panel.

Mount the LED counter board. I've cut out a small piece of transparent red perspex between the front panel and LED board. It's just held in place by slightly loosening the standoffs between the board and front panel.

Put the front panel in place, mark and drill the mounting holes for the gang switch and single switch. I'd already pre-determined the height I wanted with standoffs for the gang switches when I was designing the front panel.

Mount the signal generator board in place as well. I mounted it to one side so I'd have easy access for calibration if needed.

Also drill and mount the rear panel DC and BNC connectors.

Make up the wiring looms for the pots, switches etc from the boards using either hookup wire or ribbon cable. Assemble to female connector ends to connect to the main boards. I've found it's best to fold the tab over with needle nose pliers and put a little solder on them to keep the wires falling out. Then press them into the black connectors.

Start by soldering up the pots.

While they're only short runs, it's still good practice to use shielded cable for output connectors. Wire the rotary signal selector switch. Now connect up the out BNC connectors to the int/ext switch and board connector wires.

Once that's complete, wire up the gang switch.

Hook up the power switch and power cable to the main boards. Use small spade connectors to connect to the switch. I've just attached the wires to the main board sockets as the DC connectors hadn't arrived as of writing (hence why nothing has been cable tied yet in the photos). I'll retrofit them when they do arrive

To finish off with, put all the knobs onto the front panel.

Because you should have checked each individual board before hand, everything should be functioning as it should.

Check that the front LED meter is measuring something (that's at least a good sign). Select a frequency range and make sure the measurement changes. Your can also check your int/ext switch/inputs by hooking up an external signal generator and seeing if it measures external signals.

Finally, hook it up to an oscilloscope and make sure you're getting the correct signal types, and that all the controls behave as they should. The great thing about wiring with connectors is if it is working in reverse, simply turn the cable connector around!

There is a calibration procedure for the signal generator board that should be included when you buy the kit. You'll need an oscilloscope to do this, but this is an excerpt from instructions (or there about's):

Connect an oscilloscope to the square output. Adjust the DUTY control to 50%, then switch to sine. Adjust R2 & 3 to sine wave crest to minimise distortion. Once R2 & 3 are set, they shouldn't need adjusting again. To output a saw-tooth wave, select Tri. Adjust DUTY control and convert triangle into saw-tooth.

Hopefully everything's working for you.

All in all I think the project came out extremely well. While you could probably buy something more accurate for considerably more money, it was definitely a fun build (although it's been sitting on the bench for quite a while)!

Sometimes builds don't go right first go and end up being better for it. This project was one of those.

The first photo is trying to mangle all the controls onto the front of a smaller box (I've got heaps of these boxes as they're cheap and generally fit most test gear type projects fairly well). I tried every which way and even took the time to set it out. In the end it was too hard and confusing using toggle switches and wanting to have a large knob for frequency control on the front. Plus the lettering is getting old and not sticking well these days. That's when I stumbled on front panel software which I'll probably use for other projects going forward.

Also on the first attempt, I found out that my new larger drill bits are way too savage. I ended up cracking the edge when I was drilling one of the BNC holes when it grabbed. From then on, I only used up to an 8mm bit and used a reamer to get the final larger hole sizes.

The second photo I almost had it right, until I started assembling and realised it would be better to switch all of the signal types instead of having two separate outputs. Then I could mount one on the back for a hidden connector. It de-cluttered the front a little too I think. As I didn't need one of the front panel holes now, it was no sweat removing one of the holes using the front panel software. It easily covers up any mistake (design change)!