diff --git a/README.md b/README.md
index 6a7b631..fed9c1e 100644
--- a/README.md
+++ b/README.md
@@ -47,7 +47,8 @@ Please also see the [official examples](https://github.com/micropython/micropyth
   4.15 [Date](./README.md#415-date) Small and simple classes for handling dates.  
   4.16 [Greatest common divisor](./README.md#416-greatest-common-divisor) Neat algorithm.  
   4.17 [2D array indexing](./README.md#417-2d-array-indexing) Use `[1:3, 20]` syntax to address a 2D array.  
-  4.18 [Astronomy](./README.md#418-astronomy) Derive Sun and Moon rise and set times, moon phase.  
+  4.18 [Astronomy](./README.md#418-astronomy) Derive Sun and Moon rise and set times, moon phase.
+  4.19 [Tone detection](./README.md#419-tone-detection) Goertzel algorithm.
  5. [Module Index](./README.md#5-module-index) Supported code. Device drivers, GUI's, utilities.  
   5.1 [asyncio](./README.md#51-asyncio) Tutorial and drivers for asynchronous coding.  
   5.2 [Memory Device Drivers](./README.md#52-memory-device-drivers) Drivers for nonvolatile memory devices.  
@@ -394,6 +395,12 @@ This module enables Sun and Moon rise and set times to be determined for timing
 applications or for lunar clocks. Moon phase can also be accessed. Designed to
 work at global locations and timezones. See [docs](./astronomy/README.md).
 
+## 4.19 Tone detection
+
+This module may be used for detection of audio tones. It uses the Goertzel
+algorithm which is effectively a single-bin Fourier transform. See
+[docs](./goertzel/README.md)
+
 # 5. Module index
 
 This index references applications and device drivers that I have developed, in
diff --git a/goertzel/README.md b/goertzel/README.md
new file mode 100644
index 0000000..bf05268
--- /dev/null
+++ b/goertzel/README.md
@@ -0,0 +1,5 @@
+# Tone detection
+
+The `goertzel3.py` implementation was written for the Pyboard but could be
+adapted for other platforms. It includes test tone generation. See code
+comments for more details.
diff --git a/goertzel/goertzel3.py b/goertzel/goertzel3.py
new file mode 100644
index 0000000..110d38d
--- /dev/null
+++ b/goertzel/goertzel3.py
@@ -0,0 +1,83 @@
+# Tone detection using Goertzel algorithm.
+# Requires Pyboard 1.x with X4 and X5 linked.
+
+from array import array
+import math
+import cmath
+from pyb import ADC, DAC, Pin, Timer
+import time
+import micropython
+import gc
+micropython.alloc_emergency_exception_buf(100)
+
+# When searching for a specific signal tone the Goertzel algorithm can be more efficient than fft.
+# This routine returns magnitude of a single fft bin, which contains the target frequency.
+
+# Constructor args:
+# freq (Hz) Target centre frequency
+# nsamples (int) Number of samples
+# spc (int) No. of samples per cycle of target frequency: sampling freq = freq * spc
+# Filter bandwidth as a proportion of target frequency is spc/nsamples
+# so 1KHz with 100 samples and 10 samples per cycle bw = 1KHz * 10/100 = 100Hz
+
+# .calc() computes magnitude of one DFT bin, then fires off a nonblocking acquisition of more data.
+# Depending on size of sample set, blocks for a few ms.
+
+class Goertzel:
+    def __init__(self, adc, nsamples, freq, spc, verbose=False):
+        if verbose:
+            print('Freq {}Hz +- {}Hz'.format(freq, freq * spc / (2 * nsamples)))
+        self.sampling_freq = freq * spc
+        self.buf = array('H', (0 for _ in range(nsamples)))
+        self.idx = 0
+        self.nsamples = nsamples
+        self.adc = adc
+        self.scaling_factor = nsamples / 2.0
+        omega = 2.0 * math.pi / spc
+        self.coeff = 2.0 * math.cos(omega)
+        self.fac = -math.cos(omega) + 1j * math.sin(omega)
+        self.busy = False
+        self.acquire()
+
+    def acquire(self):
+        self.busy = True
+        self.idx = 0
+        self.tim = Timer(6, freq=self.sampling_freq, callback=self.tcb)
+
+    def tcb(self, _):
+        buf = self.buf
+        buf[self.idx] = self.adc.read()
+        self.idx += 1
+        if self.idx >= self.nsamples:
+            self.tim.deinit()
+            self.busy = False
+
+    def calc(self):
+        if self.busy:
+            return False  # Still acquiring data
+        coeff = self.coeff
+        buf = self.buf
+        q0 = q1 = q2 = 0
+        for s in buf:  # Loop over 200 samples takes 3.2ms on Pyboard 1.x
+            q0 = coeff * q1 - q2 + s
+            q2 = q1
+            q1 = q0
+        self.acquire()  # Start background acquisition
+        return cmath.polar(q1 + q2 * self.fac)[0] / self.scaling_factor
+
+# Create a sinewave on pin X5
+def x5_test_signal(amplitude, freq):
+    dac = DAC(1, bits=12)  # X5
+    buf = array('H', 2048 + int(amplitude * math.sin(2 * math.pi * i / 128)) for i in range(128))
+    tim = Timer(2, freq=freq * len(buf))
+    dac.write_timed(buf, tim, mode=DAC.CIRCULAR)
+    return buf  # Prevent deallocation
+
+def test(amplitude=2047):
+    freq = 1000
+    buf = x5_test_signal(amplitude, freq)
+    adc = ADC(Pin.board.X4)
+    g = Goertzel(adc, nsamples=100, freq=freq, spc=10, verbose=True)
+    while True:
+        time.sleep(0.5)
+        print(g.calc())