Frequency Division with Amplitude Restore: Construction

Printed circuit board

Probably the most difficult part of the project is making the printed circuit board (PCB).

The ‘traditional’ method is to print the PCB layout on good quality tracing paper. Make sure that your printer has not altered the dimensions before continuing. If it has, you will have to scale the layout. The board size should be 4.80 * 2.30 inches. You will need a good black, not grey image. The image is then used to expose a photo sensitive coated PCB using an ultraviolet light box. The PCB is then ‘developed’ to remove the unwanted photo sensitive mask and then etched in ferric chloride. The board will then need to be drilled.

An alternative is to use an iron-on PCB mask. This is first laser printed and then the image is transferred to a plain copper PCB with a domestic iron. You may need some practice to perfect this technique.

You will find many good tutorials on PCB manufacture on the Internet.

If you don’t want to follow the DIY route, there are a number of companies that will produce PCBs from artwork, although the costs for a one-off may be quite high. It’s best if you can ‘club together’ and make a bulk purchase.

As an alternative, you may find that someone at your local college or radio or electronics club will be willing to help, especially for a donation to funds.

A final option is to visit the Fab Lab in Manchester (England). There may be also be a Fab Lab near you. Not only will Fab Lab help you make the PCB, they will provide help and guidance with all stages of the construction. Many of the facilities are free of charge, you only pay for materials. http://manchesterfablab.manufacturinginstitute.co.uk/

 

Components

It is unlikely that your local high street electronics shop will have all the components in stock, but they can probably provide a wider range by mail order.

There are at least three big electronics companies in the UK that primarily supply to the trade but that will also supply to private constructors. They are likely to have everything you need in stock, and offer next day delivery. Beware of post and packing costs for small orders, but larger orders are free.

The cheapest source of components is the Internet, specifically Ebay. You will have to order from several suppliers, and sometimes in multiple quantities, but you may still find this cheaper than buying a single item from an established company. Check where the supplier is based; some of the best bargains come from the Far East, but there may be long delivery times.

It is best to buy the integrated circuits (ICs) from a reputable source as counterfeiting is becoming a major problem.

Make sure that the preset resistor (R28) is the type with all three legs in-line. The type with offset legs will not fit on the board.

Capacitors tend to come with 1/10 inch (100 mil) or 2/10 inch (200 mil) lead spacing. For ease of construction, try to get the ones with the right spacing. See the components list for details.

The Schottky diode D1 can be any Schottky signal diode, for example a BAT43. Although D2 is marked as a 1N4148, another Schottky diode is probably better.

The TL062 ICs (U2 and U3) have been chosen for their low current consumption. However, they are less responsive at higher frequencies. The peak response is approximately 45KHz and they will give good results to over 65KHz. Adding 150pf capacitors in parallel with R5, R8 and R11 will raise the peak response to approximately 55KHz with good results to over 90KHz, but with less performance at lower frequencies. Alternatively LF353 ICs can be used (without the extra 150pf capacitors). They have improved high frequency response, but at the expense of battery life. The pin configuration is identical. The peak response is approximately 75KHz and they will give good results from about 45KHz to over 125KHz.

If you use a MEMS microphone, R12 may need to be reduced to say 27K as the output of the microphone is much higher than an electret. R12 may need some experimentation depending upon the microphone used (the higher the resistance, the greater the gain). Also note R4, C26 and ZD1 vary depending upon the choice of microphone.

Building the board

The board is designed to fit into the lid recess of a Maplin code BZ73Q box. The first thing to do is to shape the four corners of the board to fit around the lid screw fixings. Although it is tempting to drill the PCB to get the basic shape, this is very likely to crack the corners of the board. It is better to use a small hacksaw and flat & round files. This takes longer, but reduces the likelihood of damage.

Start by fitting the wire links and the small components first, followed by the larger ones. The resistors and normal capacitors can be fitted either way round, but the electrolytic capacitors, diodes, transistor and ICs must be fitted the correct way round. Be particularly careful with the fitting of U5. You will also need to link pins 2 and 3 of PL4.

It is best to use sockets for the ICs so they can be removed and replaced.

Customising the box

Firstly print the front panel artwork on plain paper, and check that it is the correct size. Cut out the artwork and temporarily tape it to the front of the box, making sure it is correctly placed. Use a sharp point to mark the centre point of all the holes.

The artwork can now be removed. Start with a small drill and progressively use larger ones until each hole is the required size. The holes for the speaker should be a size larger than the ones in the front panel so they do not show through the finished front panel.

Now print the front panel artwork on photo paper. Carefully check the size as some photo printing software / printers can change the dimensions. If necessary, you will have to scale the image.

The next stage is to cover the back of the artwork with thin double sided sticky tape. The type used for sticking down carpets is ideal. The panel is then cut to size; a steel rule and craft knife is best. The holes for the speaker and volume control, LED etc are then cut with appropriate sized hole punches. It’s probably best to have a practice first as they can be difficult to position precisely.

Finally, the backing of the double sided tape is removed and the panel carefully fitted into position.

 

Testing

There are a number of components, such as the microphone and speaker that are external to the PCB and should be connected with short lengths of wire. A dab of hot melt glue where the wires leave the PCB is useful to stop the wires snapping during testing and use. Battery connectors are particularly prone to breaking.

The connections for the microphone will depend on the type in use. Very thin screened wire is best to minimise the risk of picking up unwanted noise. Note that although some electrets work well at ultrasonic frequencies, others will be found to be quite unresponsive.

First carefully check the PCB for any solder bridges. Insert IC1 and connect the battery. The voltage at the output of IC1 (pin 6) should be approximately half the battery voltage.

Disconnect the battery and insert the other ICs. Reconnect the battery. Depending upon the setting of R28 there may be an immediate noise from the speaker. If not, try jangling a set of keys close to the microphone. If all is well, they will produce a loud noise from the speaker.

To set R28, turn it clockwise or anticlockwise (depending upon which way round you have mounted it) until there is continuous sound from the speaker. Back off the setting until the noise just stops. This should be done in a quiet setting away from anything that may be generating ultrasonic sound. Fine adjustment is best done in the environment where you will be listening to bats.

 

Finishing off

The PCB is held in the lid of the box with several small dabs of hot melt glue. A battery compartment can be made using two off cuts of PCB or sturdy plastic. There are pre-formed slots in the box and by using the right ones, the battery will be a snug fit. Do this before gluing in the speaker to ensure everything fits.

The microphone should be mounted outside the box to minimise the chances of feedback from the speaker. An electrical rubber grommet super glued to the box makes a good mounting, and a small hole can be drilled to feed the wires into the main enclosure.

Finally, it is a good idea to place a piece of sponge foam over the speaker and fill other cavities in the box, again to reduce acoustic feedback.