What is the use of capacitor in this circuit? ......And why don't we get an electric shock when we

Alec_t

Joined Sep 17, 2013
10,617
Welcome to AAC.
I don't see how the LEDs can turn on for any significant time unless the cap is a supercap. The cap will charge up when the first LED is switched on. Thereafter there will be no LED current.
 

Thread Starter

Sukanya1

Joined Jan 30, 2017
6
Welcome to AAC.
I don't see how the LEDs can turn on for any significant time unless the cap is a supercap. The cap will charge up when the first LED is switched on. Thereafter there will be no LED current.
What I have understood till now is that when step down transformer converts high voltage to low volt the diode converts a.c. To dc ...when we touch the bottom most wire of the sensor and along with it touch some other wire of at least one of the leds the circuit gets completed n d led glows....but I don't come to know then what is the use of cap there .....n why don't we get an electric shock when we touch the sensor wire
 

AlbertHall

Joined Jun 4, 2014
8,847
I don't see how the LEDs can turn on for any significant time unless the cap is a supercap. The cap will charge up when the first LED is switched on. Thereafter there will be no LED current.
Yes, it's not going to work.
Maybe the capacitor was supposed to be connected as reservoir?
Or without the cap, but with its leads shorted.

n why don't we get an electric shock when we touch the sensor wire
Because the transformer has reduced the available voltage to below that which will produce a shock.
 

wayneh

Joined Sep 9, 2010
16,129
... why don't we get an electric shock when we touch the sensor wire
It would be mostly because the voltage is low, as noted. Think about a 12V car battery. You can grab both electrodes and feel nothing. Another reason is that, to get shocked, there needs to be a complete circuit including your body. Touching just one side of a DC voltage, even a fairly high one (say, 100V), won't shock you if you're not connected to the opposite pole. With very high voltage (say, >1000V), enough current can flow through unexpected paths such as your shoes, or the air, to complete a circuit.
 

Thread Starter

Sukanya1

Joined Jan 30, 2017
6
Yes, it's not going to work.
Maybe the capacitor was supposed to be connected as reservoir?
Or without the cap, but with its leads shorted.


I have a project made on this circuit n it works when we touch any of the sensor wires along with the bottom most wire.So the person's body is also included in the circuit n the respective leds glow
 

wayneh

Joined Sep 9, 2010
16,129
Maybe I've confused myself but Im actually beginning to think this could work. It's an AC circuit. Pulsing DC enters the soil and comes back from the sensors as pulsing base current to the transistors. The cap limits the current similar to a watt-less dropper power supply.
 

AnalogKid

Joined Aug 1, 2013
8,231
This circuit was made by an expert
Creative thinker - yes.
Expert - no.

There are many threads about monitoring soil moisture content on this and other fora. Two common conclusions are a) it is very difficult to get consistent results; and b) the sensing current can be any wave shape as long as it is true AC (with zero DC component). This prevents galvanic corrosion of the electrodes.

Wayne beat me to some of this; fingers are slowing down...

The circuit misses on both points. The threshold used to detect each moisture level is a single transistor base-emitter junction. This is not a very "sharp" comparator, so the LED will wander up in brightness as moisture increases. Also, the moisture level needed for a specific brightness will change depending on the ambient temperature around the transistors. Worse that that is the sensing current. Half-wave rectified AC has a significant DC component, and will cause electrochemical problems around the probe wires.

You don't say where you are located, so we'll assume the powerline frequency is 60 Hz. This is almost certainly wrong, but that's what happens. At half-wave rectified 60 Hz, the capacitor represents an impedance of 12 ohms. How certain are you that the schematic is correct?

ak
 
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