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Although if you are just reading this for the first time, most of this will look like complete rubbish, I can guarantee that it is all true, and most of it even works!
In this section I shall try and describe, a accurately as possible, how the circuit works. The circuit basically does the job of and A to D converter.
A to D converters are available in an integrated circuit. This would have made life much easier if I could of used one of these but in the end, after significant research I found that they would be too expensive for the small budget we are working with.
The circuit that I have designed instead is much cheaper to make so in that respect it is better than simply using an IC.
Basically, the circuit uses a D to A converter to produce a ramp voltage which is compared to the voltage to be tested. The flow diagram below, I think, explains the basic functions of the circuit more clearly.
For the purposes of testing the circuit, I had to generate a 4-bit binary count using components as the chip has not yet been programmed. I have used the circuit below to do this job. This circuit is a standard way of producing pulses. I used this in conjunction with a 4-bit binary counter tin order to produce the count.
From the binary count I have had to use a transistor arrangement as I found during testing that the current output from the chip was no high enough to give a sufficient supply to the op-amp. This arrangement can be seen on the diagram.
After the transistor arrangements, the output voltages are added together by a summing amplifier. This leads to a ramp voltage and this completes the D to A converter circuit.
This output is then compared to the line voltage using another op-amp, this time as a comparitor. When the ramp voltage becomes higher than the line voltage then the overall output turns high.
This is a signal which the MCU will receive. When it receives this signal, it will record the 4 bit binary output which is given out and convert this into a voltage. This will be written into memory and also displayed on the screen. This is the voltage on the line.
There is another op-amp in the circuit. This is set up so that it has unity gain. Its only purpose is to provide the high impedance that the circuit requires. This is so that the circuit does not draw large current from the lines.
The zener diode in the circuit is there so that there is a constant supply voltage of 6.1V. A 12V supply has had to have been adapted as the 5V supply was not enough to compensate for the voltage drop over the internal resistance’s of the op-amps in the circuit.
This circuit will need to be produced twice for each unit as both the Tx+ and Tx- lines need to be monitored.
The voltage regulator circuit is to be integrated into the battery monitor circuit. It will ensure that the voltages cannot rise above a specified amount, therefor no damage can occur to the circuit because of freak voltage spikes.
The high impedance operational amplifier circuit is used to stop the unit from causing a power drop across the lines when it is connected up to it. It stops this voltage drop from occurring by producing a huge resistance (unity gain). This means that a very low current will flow.
A the unit will be plugged across a line which is susceptible to huge voltage spikes, for example, a lightning strike, the unit must include some sort of built in protection so that in the unlikely even that this should happen, the unit will not be damaged (and more importantly, nobody will get electrocuted.)
We used PTC (Positive Temperature Coefficient) resistors for this purpose because the resistance of these components increases with the temperature they are exposed to. This means that as the current through the resistor increases, the heat they are exposed too also increases. This will cause a snowball effect because now that the resistance has increased, the temperature will also go up, causing a resistance increase, this will continue until either the resistors burn out and break the circuit, or until the spike stops and the resistors can cool, allowing the circuit to go back to normal. If the resistors do blow, they have served their purpose and they can be replaced for very little extra cost.
As well as the resistors in the circuit, to further protect against damage caused by voltage spikes, we have inserted zener diodes into the circuit. This will mean that because they are in reverse bias, they will limit the voltage to a level which we consider to be acceptable. In this case we have used the diodes so that if the voltage increases too high and they can no longer limit this, instead of allowing the circuit to be damaged, they will blow in a similar manner to the resistors. Therefor the worst that can happen to the unit if it is exposed to this sort of power surge is that four of the discrete components will need to be replaced, we estimate that this will cost a little under £1. Not bad considering the unit will be saved.
Our unit is to be booted up (started) by power from two 3.6v batteries. It would be inconvenient to have to remove the batteries every time they needed to be charged up, it would also be inconvenient to have to plug it into an external power source to have to charge the battery (similar to a mobile phone). Instead of either of these charging types, we are going to use internal charging with fixed batteries. That is, we are going to leave the batteries in-situ permanently and they will be charged from the power lines while the unit is in use. Into this circuit we have integrated a monitoring circuit so that we can tell when the power of the batteries is beginning to drop below a level where the unit will start up.
To tell when the battery voltage drops below 6.5v (a minimum useful voltage for the unit), we have used a zener diode and an operational amplifier. The zener diode provides us with a constant voltage of 3.3v into the operational amplifier, because we are using two batteries, we a also use a potential divider to supply an input to the operational amplifier of 3.3v or higher, when the total voltage from the batteries is 6.5v or greater.
When testing this circuit, we discovered a problem with hysterysis, so to compensate for this, we inserted a diode and a resistor. When connected to the operational amplifier, this formed a feedback loop which solved this problem.
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