# What device controls whether a signal is high or low

In circuitry, there is always inputs in circuits (for example: input A and B into an AND gate), but what exactly allows these inputs to switch states?

I think the best way to illustrate what I have in mind is the following:

Where basic is the input to the device

I can see how a person can switch the pivot between high and low but clearly that is not the case for electronics. In essence I am curious what the Basic (the most fundamental device possible) is. At the moment my best guess is it has something to do with the clock but I cannot expand on that idea.

I believe what I am describing is a device that takes in no physical input signals but outputs either a high or low depending on what is asked of it. And yes I understand that is an input in it's own sense.

I have experience with coding down to assembly language and the basics of computer architecture just for reference.

EDIT:

changing my notation of "base" to "Basic" to avoid confusion with transistors.

Also the command to change high or low is coming from code (for example digitalWrite(pin1, HIGH) in arduino)

EDIT 2:

My metaphor to what I am thinking. A person is using an old school computer with mechanical switches to manually enter any signals that are needed for the computer to do something. The person in this case is the switch in figure 2 and I am curious what is the modern day equivalent for this person

• Logic devices are built from transistors these days. Before those they were built from electromechanical relays. Is this what you are asking? – Justme Sep 16 '20 at 18:13
• Your question is not clear at all. First, a "base" is one terminal of a bipolar transistor. Your re-purposing of that word will bring bad results. Please describe things more clearly. Second, your 2nd drawing shows an input switchable g=between 5 V and 0 V, with the result going to the rest of the device. What is the question here? – AnalogKid Sep 16 '20 at 18:16
• "No physical input signals but outputs either a high or low depending on what is asked of it" <- How does it know what is asked of it? Does it read your mind, does it randomly go high when it feels like it, or do you have to press a button or something? – user253751 Sep 16 '20 at 18:21
• Possible duplicate of electronics.stackexchange.com/questions/53142/… – MarkU Sep 16 '20 at 18:22
• Your program code is stored in the EPROM/Flash memory as low and high voltages. The operation of the processor effecitvely transfers these voltages from memory to the output register of the microcontroller, and transistors in the output register will make the output High or Low in response to these voltages. – Peter Bennett Sep 16 '20 at 19:05

We have an electronic counter using various latches shown here as SR (set-reset) latches (for want of something better in the component library). On power-up we need to reset the counter to zero to avoid it starting at a random number.

simulate this circuit – Schematic created using CircuitLab

Figure 1. A power supply, on-off switch and power-up reset circuit to reset all the latches.

How it works:

• When SW1 is open (as shown) C1 will discharge through R1 and the rest of the circuitry. The voltage across it will be 0 V.
• When SW1 is closed to power up the circuit NOT1 and LTCH1 to LTCH3 will power up. The latches could power up with their Q outputs high or low but we don't want that.
• Since C1 is initially at 0 V, the input to NOT1 will be at 0 V (low) and, since it is an inverter, the output will be HIGH. This is fed to the R (reset) input of each of the latches and this will force their Q outputs LOW.
• R1 - C1 forms a time delay with a time constant τ = RC = 10k × 1μ = 10 ms. The voltage will rise exponentially and after about 10 ms will reach the HIGH threshold of NOT1 causing it to read the input as logic '1'. The output will switch to logic '0' removing the reset from the latches and enabling them to count when their 'S' inputs are triggered.

The above should give you an idea of how we can have the input of one device controlled by an external event (SW1 closing) and that controlling other devices by switching internal signals.