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Capacitance Meter

capacitance meter


This kit is supplied by Sparkfun Electronics (www.sparkfun.com) of Boulder, Colorado, and costs around £15 delivered. The meter measures from 1 picofarad to 500 microfarads with an accuracy of up to 1% and less than 2%. The meter works by measuring the time taken to charge the unknown capacitor. All other things being equal the time taken is directly proportional to the capacity.

The capacitance meter kit includes a good quality silk screened printed circuit board (PCB) and all the necessary components. The kit is straightforward to construct but not ideal for a first construction project as there are minimal assembly instructions supplied. To correct this I have created step by step instructions below.

capacitance meter
(click on any image for a larger version)

Before starting to construct the kit check against the parts list to ensure that all the necessary components have been supplied by Sparkfun. At the website there are a number of images relating to the project, it is worth downloading and printing the pictures of the assembled board as an aid to locating component position and orientation where appropriate. Note that the parts supplied are to get the basic meter up and running, in addition to the supplied parts I would suggest that a dual in line socket is used for the Programmable Integrated Circuit (PIC), U1, (see Maplin FZ57M or BL21X). I would als recommend replacing R1 to R8 as these resistors control the current going through the LED segments and therefore the brightness of the display. It has been reported that the supplied 1k5 resistors are too high a value resulting in a dim display. I have found that replacing them with 680R gives a reasonable brightness.

Next think about how you want to construct the meter, do you want the power on/off switch, zero switch and power socket on the PCB? If you are going to put the meter in a box then it will be easier to use components that mount on the box rather than the PCB and then run wires to the board. Consider using a PP3 battery to power the meter which will make it completely self contained, remember that you will need to buy a battery connector (a few pence at Maplin).

If you are going to mount the meter in a box then it makes sense to use the PCB as a template to mark out your box. This is easily and safely done before the PCB has any components mounted on it. Marking out the cut-out for the display might be best left until the display is mounted on the board so the exact position can be determined. When using either metal or plastic boxes I usually cover the box in good quality masking tape to protect it. I then use a sharp pencil for marking out the position of holes and cut-outs. When choosing a box be aware that dimensions given in the catalogues are usually external.

Construction

capacitance meter


First off identify the components and arrange them so that they are easily picked up for insertion in the PCB. Use the screen printing on the PCB or one of the pictures to determine component location. The silkscreen indicates components that have not been supplied, these are for various options and development. In the instructions below it is assumed that you have the silkscreen printing uppermost so that WWW.JYETE.COM is in the bottom right hand corner.

Before starting to assemble the meter read through the instructions carefully and ensure that they are clear to you. Although not essential it is useful to have a solder sucker and a digital multi-meter both of which can be had for a few pounds each.

1. Start off by inserting the resistors R1 to R8, (located below the display area, DIG1). I used 680R, colour code, blue, grey, brown.

Bend the resistor leads outwards to hold them in place and then turn the board over. Solder the leads at the board and cut off surplus leads. Always check your solder joints, they should be smooth and shiny, any dull or crinkly uneven joints should be re-soldered.

2. Next insert and solder the remaining resistors (all located below the display area, DIG1):

R9, 120R, colour code Brown, Red, Black.
R11, 3M3, colour code Orange, Orange, Green.
R12, 3K3, colour code Orange, Orange, Brown.
R13 & R14, 10K, colour code Brown, Black, Black, Red.
R15 & R16, 39K, colour code Orange, White, Black, Red.

NOTE: There should be no resistor in the positions for R10, R17 & R18

3. Insert socket J5 and turn the board over, make sure the socket is still inserted and solder.

4. Now insert and solder the 12MHz crystal, Y1. Again bend the leads to hold in place, turn the board over, solder the crystal and trim off excess leads.

5. Insert capacitors C1 & C2, 22pf, these are the small brown discs with 22 on the side and a black band on the top of the capacitor. Bend the leads to hold in place.

6. Now insert capacitors C3, C4 & C7, 0.1uf, small brown discs with 104 on the side. C3 is inserted to the left of the crystal and left of C1. C4 & C7 are located in the middle of the board. Bend the leads to hold the capacitors in place and then turn the board over. Solder the capacitors and remove excess leads.

7. Insert diode D1,1N4001, which is located near the top right hand corner of the PCB. The white band of the diode goes downwards matching the white band on the silkscreen. Bend the leads to hold the diode in place and turn PCB over. Solder the diode and remove excess leads.

8. Insert U2, 7805 regulator, align the flat side of the regulator with the flat on the silkscreen. Bend the leads outwards to hold U2 in place, turn the board over and solder. Remove excess leads.

9. Now insert the two electrolytic capacitors, C5 & C6, these are about 10mm long and 5mm diameter, see note below. On the side of the two capacitors you will see a white stripe, the lead on that side is the negative one and the other which is longer is the positive lead. For C6, the longer lead goes in the hole facing D1 and the shorter lead faces the on/off switch SW2. For C5, the longer lead faces the row of resistors and the shorter lead, white stripe side, faces SW2. Bend the leads to hold in place then turn over and solder the capacitors, remove surplus leads.

NOTE: If you are going to mount the PCB in a box with the display flush then you will need to bend these two capacitors over as they are higher than the display. When bending the capacitors over make sure that the leads do not short together.

10. If you intend to use the on-board switches carefully insert SW1 & SW2 then turn over and solder. If you plan to mount the switches on a panel then you might want to solder pins in the relevant holes. For SW1 this will be the top two holes either side of SW1 on the silkscreen. For SW2 this will be two holes nearest to SW2 on the silkscreen. Both switches are single pole but for ease of use SW1 should be a push button.

11. If you are going to use a socket for the PIC, U1, insert it now and solder. Ensure that the indentation on the socket matches the indentation on the silkscreen (pointing to the word ZERO). Otherwise insert the PIC chip, ATmega48, and solder. Ensure the indentation on the chip faces the ZERO on the silkscreen.

12. If you intend to use the on-board power socket carefully insert it now, it goes to the right of the display area and near the diode, D1. Turn the board over and solder.

13. Finally, carefully insert the LED display with the decimal points facing towards the line of resistors. You may find that the leads need a little straightening to line up with holes in the PCB. Turn over and solder.

14. The capacitance meter is now complete and ready for testing.

15. If you intend to use off-board switches I would suggest that you wire them to the board temporarily and check the meter before putting it into a case. If you are using a socket for U1 insert the PIC.

capacitance meter

Testing your meter

Before applying power to your hard work check the assembly very carefully: Is the diode round the correct way, the regulator, PIC, display? Are the resistors in the correct places? Is the battery wired correctly, positive to the centre pin of the connector? Check for unsoldered connections and for solder splatter shorting across connections.

Connect up to a supply (8 - 16 volts DC) such as a PP3 battery and switch on, if you do happen to connect the supply the wrong way round D1 will ensure that no damage is done. Pressing SW1 should zero out any stray capacitance and give a reading of 000P after some seconds. Now you can use known capacitances to check that the meter works and auto-ranges. To check the absolute accuracy you would need capacitors of at least 0.1% accuracy but you will get a good feel for the instrument by checking with normal close tolerance capacitors such as 1% silver mica.

Faulting finding

Having followed the instructions above carefully your meter should work first time but we sometimes make a mistake or we may have been with a supplied a faulty component. In order to find out why your meter is not working you will need a digital multi-meter(DMM), they are very cheap these days and a most useful tool. Here are a few tests and checks that should show up most common faults, follow them through step by step until the fault is found. Connect the negative lead of your DMM to the negative line of your power source then:

1.Check the supply voltage with the meter switched off and then with it on, there should be very little or no variation and it should be at least eight volts. If the supply voltage is less the regulator, U2, may not function correctly.

2.Check the voltage on the white band end of D1, it should be about half a volt less than your supply voltage. If the voltage is zero then check that D1 is the correct way round. Look for a short circuit between the diode and the negative supply (ground). When checking for short circuits switch off the power supply to the meter.

3.Check the voltage at the left hand lead of the regulator, U2, it should be 5V. If it is zero check that U1 is in the correct way round. Look for a short circuit to ground on the 5 volt line. Double check that C5 is the correct way round.

4.If the supply and stabilised 5 volt line is OK make sure that the PIC, U1, is in the correct way. If you are using a socket ensure that none of the pins are bent over. Check for solder shorts around U1 on the underside of the PCB. Check that all pins have been soldered.

5.If the display is not functioning correctly check that you have the display in the correct way round. Check that there are no shorts around the display pins on the underside of the board. Check that all pins have been soldered.

capacitance meter


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