# Questions Regarding Symbols in a Schematic

So I'm building the circuit below as an exercise:

I have some questions on this schematic as I'm fairly new to reading schematics and electronics in general.

I have every component in the diagram but I'm confused on some of the symbols in diagram. What does Vin and Vs stand for?

Where would the common ground be in this circuit be and how would you be able to determine that from the diagram?

What does the BYPASS part of the schematic mean and how would I implement that in my circuit?

Also, would I be able to put a SPDT switch and LED just after Vin (assuming Vin is voltage in which I'm still unsure about)?

I would be greatly appreciative of any help, thank you.

$V_{S}$ = Supply Voltage
Sometimes you might also see $V_{S+}$ and $V_{S-}$ for dual supply op amps, although this depends on the supplier and the type of device. The table below shows the possible different notations that can be used for positive and negative supply.

$V_{IN}$ = Input voltage
This is the voltage signal that you are providing your circuit with, considering this is a typical audio amplifier, your input would be a low voltage input signal from something like a microphone

In a circuit schematic, ground is indicated wherever you see one of these symbols. All your signal grounds are linked together, all your earth ground are linked together and all your chassis ground are linked together. These are then connected to each other at a single point, but correct earthing is another topic entirely and isn't one of my strong points.
For the circuit you have there just link all the ground points together and connect it to the 0V on your power supply.

The 'BYPASS' is a pin on the actual LM386 device and is shown in the datasheet. It is to do with the internal amplifier stages of the device. Putting a capacitor between the bypass pin and ground (The datasheet recommends 10uF) prevents any noise from your power supply feeding back into your amplifier and causing any humming or buzzing you might hear from your speaker. If you have a clean power supply you don't always need one.

As for putting an LED and SPDT switch just after $V_{IN}$, it all depends on what your input voltage is. If this is a signal coming from an electret microphone then the voltage will be in the 100mV range and will not be able to light up an LED. If you let me know why it is you want the LED and what you expect it to do I might be able to give better advice.

• Thank you for your input, Doodle. Sorry for the confusion with the LED and SPDT question, I asked that with the assumption that Vin stood for voltage in (once again, fairly new to electronics). With that in mind, I think it would be wiser to put the switch and LED in series after Vs. The only reason I want an LED is to show that power is being supplied to the circuit. – Justin Berry Jun 23 '17 at 19:03
• Yeah that's right. If you just want an LED to show circuit power power it from Vs, however power the LED in parallel not series. Otherwise you'll drop the voltage of your power supply. Also don't forget the resistor for your LED – Doodle Jun 25 '17 at 17:59

Vin will be your input to the non inverting input of the op-amp. Vs in this circuit can safely be assumed to be the Voltage Supply, as it is connected to the positive power pin of the op-amp. The common ground is everything that has the ground symbol on it.