ZenteK Design

The circuit is shown in Fig 2. It's centred around a purpose-made IC, the FX315, which has been used in professional circles for many years. I have built a number of these units for my own use. It draws only 1 .5mA in use, so it's eminently suitable for handhelds, as well as mobile and base station transceivers. There's no tone alignment needed, so you can just fit and forget it for use in your geographical area. For multi-tone encode, a small DIP switch can be added to switch between any of the 40 standard CTCSS tone frequencies provided.

The circuit

The FX315 is a monolithic CMOS integrated circuit tone generator, which has been designed especially for sub-audio tone squelch systems. It's made by Consumer Microcircuits Ltd (CML) and, although it's a 'specialist' IC, it is readily available throughout the world.

An onboard oscillator circuit is contained within the IC, from which all the CTCSS tone frequencies are generated. The oscillator requires an external 1MHz crystal, so if you have one of these in your junk box you can save yourself some money. Otherwise, I'd recommend using a low cost 1MHz ceramic resonator, which is available for less than £1 .00 from component suppliers such as Maplin. I've used both crystals and ceramic resonators and either will work fine in the circuit described here.

Apart from setting the tone output level, there's no alignment needed at all in this project. That's because the IC uses digital division and filtering techniques to produce a 'rock stable' output tone frequency, derived from the 1MHZ oscillator.

To select the required CTCSS tone frequency you simply connect one or more of the IC programming input lines, D0 to D5 to ground, Table 1 gives the required program input links The DO to D5 input lines are internally pulled up to positive logic '1' by the IC, thus if any pin is left unconnected it'll be automatically linked to logic '1'. Shorting any pin to ground gives logic '0'. For example, repeaters use 71.9Hz, so all that's needed is to short D5 to 0v, I.E. IC pin 6 to DC, to provide a 0 on D5 and 1 on all other lines.

The construction

Because of the very simple circuit arrangement, making a 'one-off' PCB for the project could be regarded as overkill. The component layout isn't at all critical, I used Veroboard for my units, although even this isn't necessary as you could simply solder the components directly to the IC pins if you wish. This works well if you need to get the finished unit into a small area such as inside a handheld transceiver case. A hint here: bending the IC pin legs out horizontally and then soldering the component leads, suitably trimmed short, to these pins aids construction and achieves a thinner overall assembly, which could fit into spaces where there other-wise wouldn't be enough depth.

The IC is a CMOS type, so take suitable static precautions when you're handling it, also make sure you observe the correct polarity of the electrolytic capacitors when soldering them in.

The FX315 requires a 5V supply: (see diagram)

If you connect it to any voltage higher than 7V you risk destroying it. Many transceivers have a switched +5V line on transmit, which is ideal. If you need to use a higher voltage supply, e.g. the main 13.8V supply rail in your mobile transceiver or, say, the 7.2V or 9.6V nicad voltage in your handheld, you need to add a simple voltage regulator such as a small 78L05 three-pin regulator. Remember to add suitable decoupling capacitors on the input and output of such an IC to prevent oscillation. A suitable circuit is shown in here the capacitor values are not at all critical and you can use any suitable type from your junk box.

You'll need to feed the audio tone output to your transmitter's audio stages after the microphone audio shaping and amplification circuitry. You can not lust connect it to your mic socket. If you do, you will almost certainly find it gets filtered by your rig, the result being no CTCSS tone transmitted at the output! If your set has an internal connector for an optional CTCSS unit, this is an ideal connection point. Simply connect the tone output line from the unit to the TX tone input line on the connector. Virtually every ex-PMR rig has facility for a CTCSS unit, and a quick check of the conversion details or a circuit diagram will show you the correct tone injection point - it's usually marked 'TX sub-tone' or similar In the case of the Pye / Philips M290 and MX290 series of equipment, this is an easily-accessible pin on the front PCB-mounted facility connector.

The circuit shown generates the CTCSS tone whenever voltage is applied to it, so you can usefully link it to the +5V TX rail. If you connect it permanently, i.e. so that it's active also on receive, in rare cases and depending on your radio's circuitry, you may possibly find received signals arc modulated with the sub-tone as well it the set's VCO is modulated with the tone. If you only have a negative-going line on PTT rather than a switched positive voltage line, then to make it CTCSS unit only generate a tone on transmit PTT leave the FX315 pin 13 open circuit, and instead use a switched connection to ground on pin 12 of the FX315, i.e. the PTT line, to initiate tone generation . I'd advise adding a series diode on this line to prevent any voltage greater than 5V from your PTT line being fed back to the unit.

Level adjustment

After you've wired the unit into your transceiver, all that remains is to set the tone output level correctly. You'll need to achieve between 10% and 20% of your peak system deviation for the CTCSS tone, i.e. 250 - 500Hz deviation for 12.5kHZ spacing, and 500Hz - 1kHz deviation for 25Khz spacing, 500Hz being a good 'in-between' setting for both. The FX315 gives 0dBm output into a 600 ohm load (775mV). and depending upon your transmitter's circuitry this might already be a suitable level.

Connecting a series resistor of a suitable value, i.e. the variable pre-set VR1 in the Circuit diagram, will reduce this if needed, you should choose the value of this to suit your rig's circuitry. You might need to connect this instead as a potentiometer, i.e. with one leg grounded and the output taken from the wiper, in cases where the transmitter has a high input source impedance, although check here that your normal speech deviation isn't affected Again depending upon your transmitter's circuitry, you may or may not need a series capacitor, i.e. C5 in the circuit diagram, to isolate any DC component on the output.

For setting the level required, if you have access to a deviation meter then all well and good. If you have no test equipment then I'd suggest initially setting the output level to the lowest possible, gradually increasing it until you access the repeater satisfactorily on-air. Then note the trimmer position and increase it somewhat further, say to double the amount from this 'minimum access' point, for reliable operation.

CTCSS Circuit diagram & Parts list

C1 FX315PJ

Cl 1µF 10V electrolytic

C2 33p

C3 68p

C4 0.1µF 16V electrolytic

R1 1M

RV1 47k typical (10-100k)

Xl 1 MHz ceramic resonator or crystal

Download Data sheet PDF FX315 Adobe Acrobat reader is needed to read data sheet

1 MHz crystals available. Harwood trading Doncaster tel ..01302 351766, price is £1.00 each.

Automatic Nicad charger

by Eric Gaze, G8NKA

One of my favourite bits of kit is a handheld transceiver . If like me , you were startled at the cost of manufacturers battery packs, you probably opted for an empty case and at the next rally bought yourself enough batteries to fill it. The most common packs hold six 1.2v batteries giving a terminal voltage of 7.2v at capacities ranging from 500-900mA/hr. The commonest is 700mA/hrs and all reference to batteries in this article refer to this variety.

The calculations are simple and easily altered to to whatever type you have. To keep the batteries in top condition it is necessary to discharge them fully before recharging. This charge/discharge facility along with the built in one or 12 hour timer is the essence of this project.

Most NiCad's have their capacity written on them in the form current (mA) / time (hours). In a perfect world, if we take a fully charged battery and place a variable load across it, then adjust the load so a 800mA current flows, the battery would sit there supplying its 800mA at 7.2v for one hour. the voltage and current would fall to zero leaving the battery fully discharged.

If we then remove the load and connect the battery to a constant current source of 800mA for one hour the battery would be full. In fact its a bit more complicated, Batteries exhibit a characteristic called "memory" if for example our standard battery is repeatedly discharged to say , half its capacity, the battery remembers and over time the effective capacity falls to 400mA/hrs.
Most handhelds do not discharge the battery fully or they are recharged are before are completely discharged. The practical outcome is a gradual reduction in the working life of the battery

To overcome this problem the battery should be discharged to about 0.9V per cell before recharging starts. After several cycles this reforms the battery pack to its original capacity. It is also standard practice to over charge the battery by about 10%, ie 770mA for one hour , this is ok but it is better to take a bit longer and do the job properly.
Charging the battery at 10% of its amp/hr rate plus 10% (77mA) for 12 hours is better as the charge is low enough not to generate internal heat. Also it can be kept topped up by trickle charging, ie reducing the current to approximately 3% of its amp/hr rating(21mA) This can be maintained for about a week or so.

So now we have our charger requirements as follows.

Discharge the batteries

Charge the batteries for one hour at their mA/hr rating or 12 hours at 10% of their mA/hr rating plus 10%

Trickle charge the batteries at 3% of the mA/hr rating.

How it works

We know what is required So how is it done? referring to fig1 and ignoring the ic and surrounding components, Look at the 2 transistors and presume they are both off.
Insert the battery pack, nothing happens! connect the base of TR1 to a positive supply and it will turn on.

Think of it simply as a switch. When TR1 is switched on it puts R17 ( 8R2 ) across the batteries. Assuming the batteries are fully charged and have a terminal voltage of 8.2V ( to keep the figures simple) there will be a discharge current of 1 amp, (I = V/R) Discharging the battery in about 1 hour, in practice it will take about 50 minutes for a full battery and about 10 -15 minutes when my handheld blanks out.

Fig 1 : Circuit diagram of NiCad charger

We then remove the positive voltage from the base of TR1 , turning it of. Applying a negative voltage to to the base of TR2 (as it is a pnp device it switches on) the Darlington transistor and associated components forma simple constant current source, the current being limited by R16, Its value was chosen to give a maximum current of about 1300mA (this is about the limit of the transformer and should be adequate for any increase in battery capacity).
That is really the heart of the charger. All we need to do is build some circuitry that will monitor the voltage of the discharging battery, switch it to charge, adjust the charge current to suit a range of batteries, and build a timer for one or twelve hours, then arrange for the batteries to be trickle charged until we need them. Panic not! the ic does all of these jobs.

The ic has 3 voltage comparitors which, with suitable voltage divider resistors, monitor the state of the battery. Pin 5 of the ic would normally go to a positive temperature coefficient resistor in close proximity to the battery, so if it became hot the ic would detect this and switch of the charge current. This was dispensed with as i did not want to keep removing the batteries from the case and did not intend using the brutal half hour charge range. A simple potential divider R2 /R3 takes care of this

Pin 6 via divider R13/R10 monitors the falling battery voltage on discharge. When the battery voltage flls to about 5.4V (0.9V x 6 cells) pin 10goes low, turning of TR1 . Pin12 also goes low , turning on TR2 and the charging starts.

Transformer wiring

Auto NICAD charcher

CTCSS Encoder