# Any one can tell me what is this thing in diagram?

I was making leaser alarm system with this diagram but i don't know what this marked thing shows
If any one can help me i will be thanked to you.

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4 years ago
As others have already said, it is a sweep of the variable resistor, just drawn in a unconventional manner.

But this circuit as a whole does not make sense ...

First what is the +9V thingy? This symbol in CircuitLab denotes a voltage sum element ... which is unknown one for me and quick googling did not fix that gap of knowledge. But I assume it is possible to sum voltages that come from other sources, but there are no other sources in this circuit. So I assume this round thingy is supposed to be + terminal of a 9V battery instead.

Okay, then other thing that is wierd is that now the base of the transistor has always 9V applied to it. That is absurd, transistor has a point only if it gets somehow varied signal to its base. The rightmost part could be a voltage divider and give that signal that swithes transistor on and off depending on the light that the photocell gets. And the potentiometer (variable resistor) could be used to set the desired point of switching. But then the circuit should look something like I drew below.

I'm no expert in electronics, only a beginner, and some of my assumptions can be wrong here. But this circuit diagram seems to be made by someone really inattentive. To avoid frustration when you try to build this, please try to find alternative circuit diagrams.
bwrussell4 years ago
To expound on what rickharris said, variable resistors (potentiometers) have three terminals, the bit of wire circled connects to the "Sweep" terminal.

I'm not great with circuit diagrams so maybe someone else can comment to this but what good is a pot if you just tie the sweep to the input? Wouldn't a rheostat be more appropriate?
4 years ago
Rheostats are hard to come by these days and Pots are common - this is a way to make the Pot into the variable resistor (rheostat)
4 years ago
I see, makes sense.
4 years ago
Rheostats were commonly used as dimmers, and were generally high current/power devices.
rickharris4 years ago
The thing you circled is a bit of wire - The component close by is a variable resistor as it says on the diagram

4 years ago
+1