diff --git a/HARDWARE.md b/HARDWARE.md
index 9c6f0e4..20df0a6 100644
--- a/HARDWARE.md
+++ b/HARDWARE.md
@@ -224,7 +224,7 @@ Polling and reading follow a call and response pattern.
 
 | Name             | Status         | Interface    | Component Class    | Notes |
 | :--------------- | :------:       | :--------    | :---------------   |------ |
-| HC-SR04          | :heart:        | Digi In/Out  | `Sensor::HCSR04`   | Ultrasonic, 20-4000mm
+| HC-SR04          | :green_heart:  | Digi In/Out  | `Sensor::HCSR04`   | Ultrasonic, 20-4000mm
 | RCWL-9620        | :green_heart:  | I2C          | `Sensor::RCWL9260` | Ultrasonic, 20-4500mm
 | VL53L0X          | :heart:        | I2C          | `Sensor::VL53L0X`  | Laser, 30 - 1000mm
 | GP2Y0E03         | :heart:        | I2C          | `Sensor::GP2Y0E03` | Infrared, 40 - 500mm
diff --git a/examples/sensor/hcsr04.rb b/examples/sensor/hcsr04.rb
new file mode 100644
index 0000000..5ba64a0
--- /dev/null
+++ b/examples/sensor/hcsr04.rb
@@ -0,0 +1,14 @@
+#
+# Example of reading an HC-SR04 ultrasonic sensor.
+#
+require 'bundler/setup'
+require 'denko'
+
+board = Denko::Board.new(Denko::Connection::Serial.new)
+hcsr04 = Denko::Sensor::HCSR04.new(board: board, pins: {trigger: 6, echo: 7})
+
+hcsr04.poll(0.05) do |distance|
+  puts "Distance: #{distance} mm"
+end
+
+sleep
diff --git a/lib/denko/board/pulse.rb b/lib/denko/board/pulse.rb
index 1d4b5d8..693c20c 100644
--- a/lib/denko/board/pulse.rb
+++ b/lib/denko/board/pulse.rb
@@ -22,5 +22,9 @@ def pulse_read(pin, reset: false, reset_time: 0, pulse_limit: 100, timeout: 200)
                             value: settings,
                             aux_message: aux
     end
+
+    def hcsr04_read(echo_pin, trigger_pin)
+      write Message.encode(command: 20, pin: echo_pin, value: trigger_pin)
+    end
   end
 end
diff --git a/lib/denko/sensor.rb b/lib/denko/sensor.rb
index e844f41..3b2b028 100644
--- a/lib/denko/sensor.rb
+++ b/lib/denko/sensor.rb
@@ -14,6 +14,7 @@ module Sensor
     autoload :SHT3X,       "#{__dir__}/sensor/sht3x"
     autoload :QMP6988,     "#{__dir__}/sensor/qmp6988"
     autoload :RCWL9620,    "#{__dir__}/sensor/rcwl9620"
+    autoload :HCSR04,      "#{__dir__}/sensor/hcsr04"
     autoload :GenericPIR,  "#{__dir__}/sensor/generic_pir"
   end
 end
diff --git a/lib/denko/sensor/hcsr04.rb b/lib/denko/sensor/hcsr04.rb
new file mode 100644
index 0000000..56a5cd4
--- /dev/null
+++ b/lib/denko/sensor/hcsr04.rb
@@ -0,0 +1,33 @@
+module Denko
+  module Sensor
+    class HCSR04
+      # Speed of sound in meters per second.
+      SPEED_OF_SOUND = 343.0
+
+      include Behaviors::MultiPin
+      include Behaviors::Poller
+
+      def initialize_pins(options={})
+        proxy_pin :trigger, DigitalIO::Output
+        proxy_pin :echo,    DigitalIO::Input
+      end
+
+      def after_initialize(options={})
+        super(options)
+
+        # Receive values from echo pin.
+        echo.add_callback { |data| self.update(data) }
+      end
+
+      def _read
+        board.hcsr04_read(echo.pin, trigger.pin)
+      end
+
+      def pre_callback_filter(us)
+        # Data is microseconds roundtrip time. Convert to mm.
+        um = (us/2) * SPEED_OF_SOUND
+        mm = um / 1000.0
+      end
+    end
+  end
+end
diff --git a/src/lib/Denko.cpp b/src/lib/Denko.cpp
index 1ecaa31..96ee44d 100644
--- a/src/lib/Denko.cpp
+++ b/src/lib/Denko.cpp
@@ -122,7 +122,8 @@ void Denko::process() {
     #endif
 
     // Implemented in DenkoPulseInput.cpp
-    case 9: pulseRead             (); break;
+    case 9:  pulseRead             (); break;
+    case 20: hcsr04Read            (); break;
 
     // Implemented in DenkoServo.cpp
     #ifdef DENKO_SERVO
diff --git a/src/lib/Denko.h b/src/lib/Denko.h
index 6d483fc..ef59819 100644
--- a/src/lib/Denko.h
+++ b/src/lib/Denko.h
@@ -95,8 +95,9 @@ class Denko {
     void eepromRead         (); //cmd = 7
     void eepromWrite        (); //cmd = 8
 
-    // Included Libraries
+    // Pulse inputs (DHT and HC-SR04)
     void pulseRead          (); //cmd = 9
+    void hcsr04Read         (); //cmd = 20
 
     // Servos
     void servoToggle        (); //cmd = 10
diff --git a/src/lib/DenkoPulseInput.cpp b/src/lib/DenkoPulseInput.cpp
index 1a5e2f3..bc9925b 100644
--- a/src/lib/DenkoPulseInput.cpp
+++ b/src/lib/DenkoPulseInput.cpp
@@ -55,3 +55,34 @@ void Denko::pulseRead(){
   }
   if (pulseCount == 0) stream->print('\n');
 }
+
+// CMD = 20
+//
+// pin                : echo pin
+// val (lower 8 bits) : trigger pin
+//
+void Denko::hcsr04Read(){
+  // Store number of microseconds to return.
+  uint32_t us;
+
+  // Ensure pins are correct direction.
+  // This is handled by modeling the sensor as a multipin component instead.
+  // pinMode(pin, INPUT);
+  // pinMode(val, OUTPUT);
+
+  // Initial pulse on the triger pin.
+  digitalWrite(val, LOW);
+  microDelay(2);
+  digitalWrite(val,HIGH);
+  microDelay(10);
+  digitalWrite(val,LOW);
+
+  // Wait for the echo, up to 25,000 microseconds.
+  us = pulseIn(pin, HIGH, 25000);
+
+  // Send value.
+  stream->print(pin);
+  stream->print(':');
+  stream->print(us);
+  stream->print('\n');
+}