Practical case: Safe Hydraulic Press Control

Level: Basic — Implement a safety logic circuit requiring two simultaneous inputs to activate a load.

Objective and use case

In this practical case, you will build a digital logic circuit that enforces a «two-hand control» safety mechanism. The load (simulated by an LED) will only activate when two separate pushbuttons are pressed simultaneously, preventing accidental operation.

Why it is useful:
* Industrial Safety: Prevents operators from injuring their hands in hydraulic presses or cutting machines by forcing them to use both hands to start the cycle.
* Dual Authorization: Similar logic is used in security systems where two keys or signals are required to authorize an action (e.g., bank vaults).
* Interlock Systems: Ensures multiple conditions (e.g., Door Closed AND Start Button Pressed) are met before a machine runs.

Expected outcome:
* Rest State: The output LED remains OFF when no buttons or only one button is pressed.
* Active State: The output LED turns ON strictly when both buttons are held down.
* Logic Level: The output voltage at the gate pin reads High ($\approx$ 5 V) only during simultaneous activation.
* Visual Feedback: Immediate response from the LED indicating the «Safe to Operate» condition.

Target audience: Students and hobbyists learning basic digital logic and safety interlocks.

Materials

• U1: 74HC08, function: Quad 2-input AND gate IC.
• V1: 5 V DC supply, function: Main power source.
• SW1: Normally Open (NO) Pushbutton, function: Left-hand safety trigger.
• SW2: Normally Open (NO) Pushbutton, function: Right-hand safety trigger.
• R1: 10 kΩ resistor, function: Pull-down resistor for Input A.
• R2: 10 kΩ resistor, function: Pull-down resistor for Input B.
• R3: 330 Ω resistor, function: Current limiting for output LED.
• D1: Green LED, function: Indicator for «Motor Active» (Load).

Pin-out of the 74HC08

The 74HC08 contains four independent AND gates. We will use the first gate.

Wiring guide

Construct the circuit using the following node connections. Ensure the power supply is off while wiring.

• Power Nodes:

  • Connect V1 positive terminal to Node VCC.
  • Connect V1 negative terminal to Node 0 (GND).
  • Connect U1 Pin 14 to VCC.
  • Connect U1 Pin 7 to 0.

• Input A Logic (Left Hand):

  • Connect one side of SW1 to VCC.
  • Connect the other side of SW1 to Node A.
  • Connect R1 between Node A and Node 0 (Pull-down).
  • Connect U1 Pin 1 to Node A.

• Input B Logic (Right Hand):

  • Connect one side of SW2 to VCC.
  • Connect the other side of SW2 to Node B.
  • Connect R2 between Node B and Node 0 (Pull-down).
  • Connect U1 Pin 2 to Node B.

• Output Logic (Load):

  • Connect U1 Pin 3 to Node Y.
  • Connect R3 between Node Y and Node LED_ANODE.
  • Connect D1 Anode to Node LED_ANODE.
  • Connect D1 Cathode to Node 0.

Conceptual block diagram

Schematic

Practical case: Safe Hydraulic Press Control

      [ INPUTS / SENSORS ]                  [ LOGIC PROCESSING ]                  [ OUTPUT / ACTUATOR ]

                                            +------------------+
(VCC) --> [ SW1: Left Hand ] --(Node A)---> |      Pin 1       |
               (NO Push)          |         |                  |
                                  v         |    U1: 74HC08    |
                              [ R1: 10k ]   |    (AND Gate)    |
                              (Pull-Down)   |                  | --(Node Y)--> [ R3: 330 ] --> [ D1: Green LED ] --> (GND)
                                  |         |      Pin 3       |               (Limit)         (Motor Active)
                                (GND)       |                  |
                                            |                  |
(VCC) --> [ SW2: Right Hand ]--(Node B)---> |      Pin 2       |
               (NO Push)          |         +------------------+
                                  v
                              [ R2: 10k ]
                              (Pull-Down)
                                  |
                                (GND)

Note: U1 Power Connections (Pin 14 to VCC, Pin 7 to GND) are implied for IC operation.

Truth table

This circuit implements the Boolean function $Y = A \cdot B$.

Measurements and tests

1. Preparation: Set your multimeter to DC Voltage mode. Power on the V1 source (5 V).
2. Idle Check: With no buttons pressed, measure the voltage at Node Y. It should be $\approx$ 0 V. The LED is OFF.
3. Single Button Test: Press and hold SW1 only. Measure voltage at Node A ($\approx$ 5 V) and Node Y ($\approx$ 0 V). The LED remains OFF.
4. Single Button Test: Press and hold SW2 only. Measure voltage at Node B ($\approx$ 5 V) and Node Y ($\approx$ 0 V). The LED remains OFF.
5. Simultaneous Activation: Press both SW1 and SW2. Measure voltage at Node Y. It should read $\approx$ 3.5 V to 4.5 V (depending on the specific HC/LS logic family load and VCC). The LED turns ON.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: Safe Hydraulic Press Control

.title Safe Hydraulic Press Control

*******************************************************************************
* Component Models
*******************************************************************************

* Pushbutton Switch Model (Ideal Voltage Controlled Switch)
* Simulates the mechanical contact closing when control voltage is high (> 2.5V)
.model SW_PUSH SW(Vt=2.5 Vh=0.1 Ron=0.01 Roff=10Meg)

* LED Model (Green)
* Standard Green LED parameters
.model LED_GREEN D(IS=1e-22 RS=10 N=2 BV=5 IBV=10u CJO=10p TT=10n)

* 74HC08 Quad 2-input AND Gate (Behavioral Model for Simulation)
* Implements one gate of the IC. 
* Pins: 1=InputA, 2=InputB, 3=OutputY, 7=GND, 14=VCC
.subckt 74HC08_GATE 1 2 3 7 14
    * Behavioral Voltage Source using continuous Sigmoid function for convergence
    * Y = VCC * (Sigmoid(A) * Sigmoid(B))
    * Threshold centered at 2.5V with steep slope (k=50)
    B1 3 7 V = V(14) * (1 / (1 + exp(-50 * (V(1) - 2.5)))) * (1 / (1 + exp(-50 * (V(2) - 2.5))))
.ends

*******************************************************************************
* Main Power Supply
*******************************************************************************
* V1: 5V DC supply connected to Node VCC and Node 0 (GND)
* ... (truncated in public view) ...

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* Practical case: Safe Hydraulic Press Control

.title Safe Hydraulic Press Control

*******************************************************************************
* Component Models
*******************************************************************************

* Pushbutton Switch Model (Ideal Voltage Controlled Switch)
* Simulates the mechanical contact closing when control voltage is high (> 2.5V)
.model SW_PUSH SW(Vt=2.5 Vh=0.1 Ron=0.01 Roff=10Meg)

* LED Model (Green)
* Standard Green LED parameters
.model LED_GREEN D(IS=1e-22 RS=10 N=2 BV=5 IBV=10u CJO=10p TT=10n)

* 74HC08 Quad 2-input AND Gate (Behavioral Model for Simulation)
* Implements one gate of the IC. 
* Pins: 1=InputA, 2=InputB, 3=OutputY, 7=GND, 14=VCC
.subckt 74HC08_GATE 1 2 3 7 14
    * Behavioral Voltage Source using continuous Sigmoid function for convergence
    * Y = VCC * (Sigmoid(A) * Sigmoid(B))
    * Threshold centered at 2.5V with steep slope (k=50)
    B1 3 7 V = V(14) * (1 / (1 + exp(-50 * (V(1) - 2.5)))) * (1 / (1 + exp(-50 * (V(2) - 2.5))))
.ends

*******************************************************************************
* Main Power Supply
*******************************************************************************
* V1: 5V DC supply connected to Node VCC and Node 0 (GND)
V1 VCC 0 DC 5

*******************************************************************************
* Input A Logic (Left Hand)
*******************************************************************************
* V_ACT_LEFT: Virtual actuator (Finger) for Left Button
* Generates a pulse: ON for 50us, OFF for 50us (Period 100us)
V_ACT_LEFT CTRL_LEFT 0 PULSE(0 5 0 1u 1u 50u 100u)

* SW1: Left Safety Trigger
* Connects VCC to Node_A when CTRL_LEFT is High
S1 VCC Node_A CTRL_LEFT 0 SW_PUSH

* R1: 10k Pull-down resistor for Input A
R1 Node_A 0 10k

*******************************************************************************
* Input B Logic (Right Hand)
*******************************************************************************
* V_ACT_RIGHT: Virtual actuator (Finger) for Right Button
* Generates a pulse: ON for 100us, OFF for 100us (Period 200us)
* Timing creates overlap with Left button to test AND logic (1+1, 0+1, 1+0, 0+0)
V_ACT_RIGHT CTRL_RIGHT 0 PULSE(0 5 0 1u 1u 100u 200u)

* SW2: Right Safety Trigger
* Connects VCC to Node_B when CTRL_RIGHT is High
S2 VCC Node_B CTRL_RIGHT 0 SW_PUSH

* R2: 10k Pull-down resistor for Input B
R2 Node_B 0 10k

*******************************************************************************
* Logic Processing (U1: 74HC08)
*******************************************************************************
* U1: AND Gate processing Left (A) and Right (B) inputs
* Connections: Pin1=Node_A, Pin2=Node_B, Pin3=Node_Y, Pin7=0(GND), Pin14=VCC
XU1 Node_A Node_B Node_Y 0 VCC 74HC08_GATE

*******************************************************************************
* Output Logic (Load)
*******************************************************************************
* R3: Current limiting resistor (330 Ohm)
R3 Node_Y Node_LED_ANODE 330

* D1: Green LED Indicator (Motor Active)
* Anode to R3, Cathode to GND
D1 Node_LED_ANODE 0 LED_GREEN

*******************************************************************************
* Simulation Commands
*******************************************************************************
* Transient analysis for 250us to cover full truth table sequence
.tran 1u 250u

* Print directives for logging signal states
.print tran V(Node_A) V(Node_B) V(Node_Y) V(Node_LED_ANODE)

.end

Simulation Results (Transient Analysis)

Show raw data table (786 rows)

Index   time            v(node_a)       v(node_b)       v(node_y)
0	0.000000e+00	4.995005e-03	4.995005e-03	2.199277e-108
1	1.000000e-08	4.995005e-03	4.995005e-03	2.199277e-108
2	2.000000e-08	4.995005e-03	4.995005e-03	2.199277e-108
3	4.000000e-08	4.995005e-03	4.995005e-03	2.199277e-108
4	8.000000e-08	4.995005e-03	4.995005e-03	2.199277e-108
5	1.600000e-07	4.995005e-03	4.995005e-03	2.199277e-108
6	3.200000e-07	4.995005e-03	4.995005e-03	2.199277e-108
7	3.600000e-07	4.995005e-03	4.995005e-03	2.199277e-108
8	4.300000e-07	4.995005e-03	4.995005e-03	2.199277e-108
9	4.493750e-07	4.995005e-03	4.995005e-03	2.199277e-108
10	4.832812e-07	4.995005e-03	4.995005e-03	2.199277e-108
11	5.162979e-07	4.999995e+00	4.999995e+00	5.000000e+00
12	5.474468e-07	4.999995e+00	4.999995e+00	5.000000e+00
13	5.779894e-07	4.999995e+00	4.999995e+00	5.000000e+00
14	6.039341e-07	4.999995e+00	4.999995e+00	5.000000e+00
15	6.320124e-07	4.999995e+00	4.999995e+00	5.000000e+00
16	6.881690e-07	4.999995e+00	4.999995e+00	5.000000e+00
17	8.004820e-07	4.999995e+00	4.999995e+00	5.000000e+00
18	1.000000e-06	4.999995e+00	4.999995e+00	5.000000e+00
19	1.022463e-06	4.999995e+00	4.999995e+00	5.000000e+00
20	1.067388e-06	4.999995e+00	4.999995e+00	5.000000e+00
21	1.157238e-06	4.999995e+00	4.999995e+00	5.000000e+00
22	1.336939e-06	4.999995e+00	4.999995e+00	5.000000e+00
23	1.696341e-06	4.999995e+00	4.999995e+00	5.000000e+00
... (762 more rows) ...

Common mistakes and how to avoid them

1. Floating Inputs: Forgetting R1 or R2 causes the inputs to «float» when buttons are open, leading to erratic LED flickering or false triggering. Solution: Ensure pull-down resistors connect the inputs to ground.
2. Confusing 7408 with 7400: The 7408 is an AND gate; the 7400 is a NAND gate. If the LED is ON when buttons are not pressed, you likely used the wrong chip. Solution: Check the markings on the IC package.
3. LED Polarity: The LED does not light up even when Logic Y is High. Solution: Ensure the longer leg (Anode) faces the resistor/IC and the shorter leg (Cathode) faces Ground.

Troubleshooting

• Symptom: LED is always ON, regardless of buttons.
  • Cause: Input pins shorted to VCC or incorrect IC (e.g., OR gate 74HC32 used by mistake).
  • Fix: Check wiring at Pins 1 and 2; verify IC part number.
• Symptom: LED is very dim when both buttons are pressed.
  • Cause: R3 value is too high or VCC is too low.
  • Fix: Ensure R3 is around 220 Ω to 330 Ω; check V1 is 5 V.
• Symptom: Circuit works for one button but ignores the other.
  • Cause: Broken switch or disconnected jumper wire on one input.
  • Fix: Use a multimeter to verify continuity across SW1 and SW2 when pressed.

Possible improvements and extensions

1. Power Interface: Replace the LED with an NPN transistor (like 2N2222) and a relay to control a real high-voltage motor.
2. Master Enable Switch: Add a third switch connected to a third input (using a 3-input AND gate like 74HC11) to act as a «Key Switch» that must be active before the two hand buttons work.

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Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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