- Astable multivibrator based on 555 timer;
- NPN transistor in CC;
- Comparator and CC references;
- Monostable circuit based on 555 timer;
- Peak detector;
The irrigation system reference schematics is bellow, followed by the moisture active sensor:
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| Irrigation system main board |
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| Moisture sensor |
Overview
The circuit is an irrigation controller.
It measures the
moisture and turns on a water pump if the soil is too dry.
To prolong the
life time of the sensor, it doesn’t work continuously but there is a 555 timer
to activate the moisture reading sometimes.
If the soil is
dry it turns on a relay that activates a water pump during a few seconds.
Blocks diagram
To simplify the circuit, it can be functionally represented by the following blocks diagram:
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| Irrigation system blocks diagram |
Astable timer:
This circuit uses an 555 timer as multivibrator astable.
At the irrigation circuit, the astable circuit is
responsible to turn on and off the moisture sensor, providing periodic sensor
readings. The astable circuit is very versatile and can be used for several
other proposes.
 |
| Astable example waveform |
All good electronics expert has a brain debugger to understand a circuit, of course for beginners is hard to do it, and as big is the circuit scariest it looks like. But a good starting point is trying to imagine the waveform at some places. For the astable circuit, it will generate a squared waveform, and the low times and high times are determined on how fast the capacitor charges and discharges. In this circuit (from the schematics mentioned before) R1 is charging and R3 is discharging. As R2 is bigger than R1, the output low time will be higher than the high time, something similar to the waveform bellow.
The best reference for all designs based on some specific
purpose IC isn’t google nether a book or magazine, the best reference is the IC
manufacturer datasheet. Take a look at the datasheet from every IC is one of
the most important practices that every electronics designer should have. If we
look at page 10 from the 555 timer datasheet there is a full explanation of how it works
and how to calculate all components to provide some desired behavior.
Moisture Sensor and Relay (NPN Transistor in CC)
In the irrigation system we have two transistors. The first one in the main board is responsible to amplify the 555 timer output to drive a relay, while the second one is the moisture sensor itself, that uses the soil as a base resistor to the NPN transistor.
There are tons of materials in books and internet explaining how a transistor works. For that matters I won't copy and paste some explanation, nether compile my favorite references, I will just be very straight forward regarding how I do approach transistors in this section.
The first step is to find out couple things:
1. Is this transistor working only as a CC amplifier? Yes/No
To find out is very simple. As a general rule that works most of times, look for capacitors. AC circuits have capacitors and because of the impedance there are more considerations to take about it.
Looking back to our main schematics above, nether the main board or sensor circuit have capacitors at the transistor circuit, it means the circuit we are learning at this section is CC. Our answer to this first question is yes.
Considering your first questions as yes, you follow to the next question. If your answer is no, your circuit is AC and not CC, so you'll need to analyze it differently from what we are seeing in this section.
2. Where is your load? Collector/Emitter
You still need to calculate all currents and voltages, but if your load is in the emitter, means you have in the load almost the same voltage from the base, in this case the transistor is a voltage follower, it works as a very simple buffer. Most of times in this configuration you don't have a collector resistor or your resistor has a very low value.
If your load is in the collector, the emitter goes directly to ground and the base resistor isn't too high (order of >100k), means the voltage in your load will be Vcc and the transistors is working as a switch that turns on and off your load.
Of course it's just an quick approach and several observations should be done, but most of application have this consigurations:
There are tons of materials in books and internet explaining how a transistor works. For that matters I won't copy and paste some explanation, nether compile my favorite references, I will just be very straight forward regarding how I do approach transistors in this section.
The first step is to find out couple things:
1. Is this transistor working only as a CC amplifier? Yes/No
To find out is very simple. As a general rule that works most of times, look for capacitors. AC circuits have capacitors and because of the impedance there are more considerations to take about it.
Looking back to our main schematics above, nether the main board or sensor circuit have capacitors at the transistor circuit, it means the circuit we are learning at this section is CC. Our answer to this first question is yes.
Considering your first questions as yes, you follow to the next question. If your answer is no, your circuit is AC and not CC, so you'll need to analyze it differently from what we are seeing in this section.
2. Where is your load? Collector/Emitter
You still need to calculate all currents and voltages, but if your load is in the emitter, means you have in the load almost the same voltage from the base, in this case the transistor is a voltage follower, it works as a very simple buffer. Most of times in this configuration you don't have a collector resistor or your resistor has a very low value.
If your load is in the collector, the emitter goes directly to ground and the base resistor isn't too high (order of >100k), means the voltage in your load will be Vcc and the transistors is working as a switch that turns on and off your load.
Of course it's just an quick approach and several observations should be done, but most of application have this consigurations:
- No capacitors, load in the emitter collector in the Vcc or Rc is low -> Current buffer; Load voltage is almost base voltage [Vload = Vbase-Vbe(usually is around 0.65)]
- No capacitors, load in the collector, emitter in ground and Rb<100k -> Switch; Load voltage is same as Vcc.
Comparator and Reference
In this circuit the comparator functions is "read" the sensor signal and define if it's lower than a reference to trigger the output. In this case if the soil is too dry the monostable circuit will be activated and a water pump will turn on to irrigate the soil.
A comparator has the output state defined by inverting and non-inverting inputs. The input with higher voltage defines the output level.
Peak detector
Explanation what it is in the circuit---------------- soon will be here
Explanation how it works------------ soon will be here
Monostable using 555 timer
Explanation what it is in the circuit---------------- soon will be here
Explanation how it works------------ soon will be here
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