Lesson 3 — Ubiquitous Circuits (The Patterns You’ll See Everywhere)
Most automation panels are not “new circuits.” They are the same few patterns repeated with different labels.
In this lesson, you’ll learn to recognize those patterns fast: pull-ups, dividers, relay drivers, latching start/stop, and 4–20 mA loops. Once you can spot them, wiring diagrams stop looking like spaghetti.
- Estimated time: 20–30 minutes
- You’ll need: Lesson 1 basics (V/I/R, AC/DC) + basic safety mindset (Lesson 2)
- Outcome: You can read common control schematics and predict what the signal should do
What you’ll learn
Five circuit “moves” that show up everywhere in automation.
Pull-up / Pull-down (Stop floating inputs)
A PLC digital input is basically a “voltage detector.” If the wire is disconnected or the switch is open, the input can become floating. Floating means “random”: it can read ON because of noise, leakage, or nearby cables.
A pull-up or pull-down resistor forces the input to a known state when the switch is open:
Pull-up: default is HIGH (ON), switch pulls it to 0 V when closed
Pull-down: default is LOW (OFF), switch pulls it to +V when closed
In automation, you’ll also see this hidden inside modules. The important skill: when you see a switch and an input, ask “what is the default state when open?”
Diagram:
Draw a 24 VDC supply at the top.
A resistor from +24 V to the input node (pull-up).
A switch from the input node to 0 V.
Label the PLC input reading HIGH when switch open, LOW when switch closed.
10-minute exercise:
You have a switch connected to a PLC input. The input sometimes turns ON by itself. What’s the most likely issue?
For the pull-up circuit: What does the PLC read when the switch is open? When it’s closed?
Bonus: If your sensor is NPN (sinking), do you usually want pull-up logic or pull-down logic?
Voltage divider
A voltage divider is the simplest analog building block:
Two resistors in series across a supply. The midpoint voltage is a fraction of the supply.
That’s how many sensors and potentiometers create a measurable signal.
If R1 is on top and R2 is on bottom (to 0 V):
Vout = Vin × (R2 / (R1 + R2))
You don’t need to memorize it. You need the intuition:
If the bottom resistor gets bigger → Vout rises
If the top resistor gets bigger → Vout falls
Draw
Vin → R1 → node (Vout) → R2 → 0 V
Label Vout. Add a note: “ADC reads Vout”
10-minute exercise:
Vin = 10 V, R1 = 2 kΩ, R2 = 8 kΩ
Compute Vout.
If R2 becomes 4 kΩ, does Vout go up or down? Then compute it.
Why is a voltage divider a bad idea to power a load (like a lamp) directly?
Coil driver + flyback diode (Protect your electronics)
Relays, contactors, solenoids: they all have coils (inductors). When you turn a coil OFF, it fights back. The collapsing magnetic field creates a voltage spike. That spike can reset PLCs, kill transistor outputs, and create EMI.
The standard fix is the flyback diode (for DC coils):
Diode is placed across the coil
It is reverse-biased during normal operation
When you turn OFF, it gives the coil current a safe path to decay
Important nuance: The diode slows the release time slightly. In most automation uses, that’s fine.
Diagram:
Draw:
a 24 VDC supply
Coil to +24 V on one side
A transistor (or PLC transistor output) switching the low side to 0 V
Diode across the coil: diode cathode to +24 V, anode to transistor side
10-minute exercise:
What happens to coil voltage when you switch it OFF without a diode?
In the diode orientation above, is the diode conducting when the coil is ON? Why?
You see a relay output (mechanical contact) switching a DC solenoid. Do you still want a flyback diode across the solenoid?
Start/Stop latch (Momentary buttons, continuous action)
This is the “classic” control circuit:
A START pushbutton is momentary (NO)
A STOP pushbutton is momentary (NC)
The machine stays ON after you release START
That “memory” is created by a seal-in contact (also called self-hold / latching contact).
In relay logic: the coil energizes, then its own auxiliary contact keeps it energized.
In PLC logic: it’s the same idea, implemented with a latch or seal-in rung.
Draw
Draw Series chain:
+24 V → STOP (NC) → [START (NO) in parallel with AUX (NO)] → Coil → 0 V
Label the AUX contact as “same relay”.
10-minute exercise:
Why is STOP normally closed? What safety behavior does that create if a wire breaks?
After you press START once, what keeps the coil energized?
If the AUX contact is wired wrong (not in parallel with START), what symptom do you get?
4–20 mA loop (The signal that survives real life)
4–20 mA is used because current is harder to “mess up” than voltage over long cables. Noise might add a little voltage, but the loop current stays almost the same.
Typical loop:
A transmitter regulates current from 4 mA (0%) to 20 mA (100%)
The receiver measures that current by placing a shunt resistor and reading the voltage across it
Common shunt: 250 Ω → gives 1–5 V (because 4 mA × 250 Ω = 1 V, 20 mA × 250 Ω = 5 V)
Diagram:
Draw:
24 VDC supply → transmitter → loop wire → shunt resistor (250 Ω) → 0 V
Put a voltmeter across the resistor labeled “AI reads 1–5 V”.
10-minute exercise:
- With a 250 Ω shunt, what voltage do you read at 12 mA?
- If the loop breaks, what current do you read? Why is that useful for fault detection?
- Your analog input expects 0–10 V. What shunt resistor would convert 20 mA to 10 V?
Quick check (2 minutes)
Set a timer for 2 minutes and solve these questions. Then check your answers.
A digital input randomly changes state when nothing moves. Most likely cause?
Floating input / missing pull-up or pull-down (or poor reference/grounding).
In a pull-up input circuit, switch open = input reads…?
HIGH (because the resistor pulls the node to +V).
What does a flyback diode do?
Gives coil current a safe path when switching OFF, reducing voltage spikes and noise.
Why is STOP usually NC?
Fail-safe: broken wire or loss of power tends to stop the machine.
Why 4–20 mA instead of 0–20 mA?
4 mA is a live “zero” that helps detect faults (0 mA can mean “0%” or “broken wire”).
Want to go deeper?
Here are some good external resources to explore:
Pull-up / Pull-down (floating inputs)
– SparkFun – Pull-up Resistors
– Electronics Tutorials – Pull-up & Pull-down explained
Voltage Divider (turn resistance into voltage)
– SparkFun – Voltage Dividers
– Electronics Tutorials – Voltage Divider Rule
Relay / Solenoid Driver + Flyback Diode
– Selecting flyback diodes for relay suppression
Start/Stop Latch (Seal-in)
– Seal-in ladder logic vs latch/unlatch
– Start/Stop circuit pattern
Sinking / Sourcing (NPN/PNP wiring intuition)
– Sink vs source logic:
– NPN vs PNP (sinking vs sourcing)
4–20 mA loops (the industrial analog classic)
– Basics of 4–20 mA current loop (incl. 250 Ω = 1–5 V):
– Fundamentals of 4–20 mA loops
– Troubleshooting a 4–20 mA loop
Common mistakes I see all the time
Wiring a switch to an input with no defined default state (floating input)
Forgetting that input commons matter (0 V reference is not optional)
Driving a DC coil with electronics without flyback protection
Building a start/stop circuit where STOP is NO (it will bite you in troubleshooting)
Assuming 4–20 mA is “voltage” and measuring it the wrong way (you need a shunt or a proper current input)
Ready for Lesson 4?
Next we’ll map these circuits to real automation hardware: relays, contactors, outputs, and how PLC I/O “really” behaves.
