How to Make Hee Haw Siren With 555 Timer

Few electronic sounds are as instantly recognizable as the classic “hee-haw” siren—the alternating two-tone wail used on vintage European emergency vehicles and classic sci-fi soundboards. While you could generate this digitally with a microcontroller, building it entirely out of discrete analog components using the iconic NE555 timer IC provides an engaging, hands-on lesson in frequency modulation (FM) and astable multivibrator circuits.

This comprehensive guide is based on the step-by-step build from Electro University’s video tutorial. Follow along to gather your parts, wire the circuit cleanly on a breadboard, and learn how two interconnected 555 timers create the distinctive warble.

Required Components & Tools

Before getting started, make sure you have all the parts gathered on your workbench [00:30]:

Component	Specification / Value	Quantity	Role in Circuit
Timer ICs	NE555 (8-pin DIP)	2	IC1 (Modulator / LFO), IC2 (Audio Tone Generator)
Resistor R1	220 kΩ (Red-Red-Yellow)	1	Sets low-frequency cycle rate on IC1
Resistor R2	22 kΩ (Red-Red-Orange)	1	Coupling resistor (feeds IC1 output into IC2)
Resistor R3	2.2 kΩ (Red-Red-Red)	1	Sets high-frequency audio pitch on IC2
Capacitor C1	1 µF electrolytic	1	Timing capacitor for IC1 (swapping rate)
Capacitor C2	0.1 µF (100 nF, marked “104”) ceramic	1	Timing capacitor for IC2 (audio pitch)
Capacitor C3	10 µF electrolytic	1	DC blocking capacitor for speaker output
Speaker	8 Ω, 0.5 W to 5 W	1	Audio output transducer
Power Source	9 V Battery + snap connector buckle	1	Circuit power supply
Prototyping	Solderless breadboard, 10 jumper wires, 2 alligator clips	1 set	Interconnections

Understanding the 555 Timer Pinout

The standard NE555 timer comes in an 8-pin Dual In-line Package (DIP). Orient the chip so that the top notch or polarity dot faces upward. Pin numbering starts at the top-left (Pin 1) and runs counterclockwise to Pin 8 [01:00]:

             +---[ \/ ]---+
     GND (1) |            | (8) VCC (+9V)
 TRIGGER (2) |   NE555    | (7) DISCHARGE
  OUTPUT (3) |   TIMER    | (6) THRESHOLD
   RESET (4) |            | (5) CONTROL VOLTAGE
             +------------+

• Pin 1 (GND): Ground reference (0 V), connected to the negative power rail.
• Pin 2 (TRIGGER): Detects when voltage drops below 1/3 VCC to set the internal flip-flop and drive the output HIGH.
• Pin 3 (OUTPUT): Delivers the generated square-wave oscillation to the circuit load.
• Pin 4 (RESET): Resets the timing operation when pulled LOW; tied to VCC to keep the timer enabled.
• Pin 5 (CONTROL): Accesses the internal 2/3 VCC voltage divider.
• Pin 6 (THRESHOLD): Detects when capacitor voltage rises above 2/3 VCC to reset the internal flip-flop and drive the output LOW.
• Pin 7 (DISCHARGE): Open-collector transistor pin used to discharge timing capacitors (not directly wired in this self-oscillating configuration).
• Pin 8 (VCC): Positive supply voltage (+9 V).

Step-by-Step Breadboard Assembly

Follow this build order carefully to establish reliable power, timing loops, and audio outputs across both chips [02:04].

1.Place Both 555 ICs on the Breadboard:

Insert the first 555 timer (IC1) across the central divider of the breadboard near the top. Place the second 555 timer (IC2) directly below it across the same center trough, leaving a few rows of clearance between them. Make sure both chips have their orientation notches facing the same direction.

2.Wire Power and Basic Straps on IC1 (Modulator):

On the upper timer IC:

• Run a jumper wire from Pin 1 (GND) to the negative breadboard bus rail.
• Run a jumper wire from Pin 8 (VCC) to the positive breadboard bus rail.
• Tie Pin 4 (RESET) directly to Pin 8 (VCC) with a short jumper wire to prevent unintended resets.
• Connect Pin 2 (TRIGGER) to Pin 6 (THRESHOLD) using a jumper wire to configure the chip for continuous self-oscillation.

3.Wire Power and Basic Straps on IC2 (Tone Generator):

Repeat the exact same power and strapping procedure for the lower timer IC:

• Connect Pin 1 (GND) to the negative rail.
• Connect Pin 8 (VCC) to the positive rail.
• Bridge Pin 4 (RESET) to Pin 8 (VCC).
• Bridge Pin 2 (TRIGGER) to Pin 6 (THRESHOLD) with a jumper wire.

4.Install Timing Capacitors:

• IC1 Timing: Insert the 1 µF electrolytic capacitor between Pin 2 of IC1 and the negative ground rail. Ensure correct polarity: the longer positive lead (anode) connects to Pin 2, while the negative side (marked with a stripe) goes to ground [03:29].
• IC2 Timing: Insert the 0.1 µF (100 nF) ceramic capacitor between Pin 2 of IC2 and the negative ground rail [03:42]. Ceramic capacitors have no polarity, so either lead can go to Pin 2.

5.Add the Feedback Resistors (Oscillation Loops):

• Place the 220 kΩ resistor across Pin 3 (OUTPUT) and Pin 6 (THRESHOLD) of the top IC [04:04]. This resistor periodically charges and discharges the 1 µF capacitor directly through the output pin.
• Place the 2.2 kΩ resistor across Pin 3 (OUTPUT) and Pin 6 (THRESHOLD) of the bottom IC [04:15]. This forms the high-speed oscillation loop for the audible tone.

6.Couple the Modulator to the Tone Generator:

Insert the 22 kΩ resistor between Pin 3 (OUTPUT) of IC1 and Pin 2 (TRIGGER/THRESHOLD) of IC2 [04:36]. This coupling resistor passes the slow switching voltage from IC1 into the timing capacitor of IC2 to drive the frequency modulation.

7.Wire the AC Decoupling Capacitor and Speaker:

• Place the 10 µF electrolytic capacitor with its anode (positive lead) into Pin 3 (OUTPUT) of IC2 and its negative cathode lead into an unused row below the chip [04:48].
• Connect a jumper wire from that negative capacitor row to one alligator clip lead, and clamp the clip to one terminal of the 5 W speaker [05:07].
• Connect a second jumper wire from the negative ground rail to the second alligator clip lead, and clamp it to the remaining speaker terminal [05:29].

8.Connect Power and Test:

Clip the 9 V battery snap connector onto a fresh 9 V battery [06:03]. Connect the red positive lead to the positive rail and the black negative lead to the ground rail [06:15]. You should immediately hear the alternating high-low “hee-haw” siren.

How the Circuit Works (Theory of Operation)

The “hee-haw” sound effect is achieved through Frequency Modulation (FM) between two cascaded oscillators [06:38]:

[ IC1: Slow Astable LFO (~1–2 Hz) ]
          |
       Pin 3 (Output: HIGH / LOW)
          |
       [ 22 kΩ Coupling Resistor ]
          v
[ IC2: Audio-Rate Astable VCO (~500–900 Hz) ]
          |
       Pin 3 (Modulated Square Wave)
          |
       [ 10 µF AC Coupling Cap ]
          v
     [ 8 Ω Speaker ]

1. IC1: The Low-Frequency Modulator (LFO)

The first 555 timer operates as an astable multivibrator cycling at roughly 1 to 2 Hz [06:58]. Because pins 2 and 6 are tied together, the 1 µF capacitor charges and discharges through the 220 kΩ resistor connected to output Pin 3.

• When Pin 3 is HIGH (~9 V), current flows into the 1 µF capacitor until it reaches 2/3 VCC (~6 V).
• The internal threshold comparator triggers, driving Pin 3 LOW (~0 V).
• The capacitor then discharges back through the 220 kΩ resistor into Pin 3 until it hits 1/3 VCC (~3 V), repeating the cycle.
• This produces a slow, steady square wave switching every half-second or so.

2. The Inter-Stage Coupling Resistor

Pin 3 of IC1 is connected through the 22 kΩ resistor to the timing network (pins 2 and 6) of IC2 [07:13].

• When IC1’s output is HIGH, it injects additional charging current into IC2’s timing capacitor, forcing it to charge faster and shifting the operating voltage baseline.
• When IC1’s output is LOW, it sinks current from IC2’s timing loop, slowing the charge cycle down.

3. IC2: The Audio Tone Generator

The second timer is configured as a high-frequency oscillator running in the human audible band (around 500 Hz to 900 Hz) using the 2.2 kΩ resistor and 0.1 µF ceramic capacitor [07:30]. Because the charging rate and voltage levels of IC2’s capacitor are pushed and pulled by IC1’s output state, the resulting tone alternates between two distinct audible pitches:

• “Hee” (Higher pitch): When IC1 output is HIGH.
• “Haw” (Lower pitch): When IC1 output is LOW.

4. DC Blocking Capacitor (Output Protection)

Pin 3 of IC2 outputs a pulsating DC square wave swinging between 0 V and roughly 8 V. Connecting an 8 Ω speaker directly across this pin would cause substantial continuous DC current to pass through the delicate voice coil, heating it up and distorting the sound. The 10 µF electrolytic capacitor acts as a high-pass filter, blocking direct current (DC) while letting the fluctuating audio frequency (AC) pass cleanly through to the speaker [07:53].

Troubleshooting & Modifications

Quick Troubleshooting Checklist

• No sound at all: Check that Pin 4 (RESET) on both chips is securely tied to Pin 8 (VCC). If Pin 4 is left floating or grounded, the timer is disabled.
• Continuous steady tone (no alternating “hee-haw”): Verify that the 22 kΩ coupling resistor between IC1 Pin 3 and IC2 Pin 2/6 is properly seated.
• Clicking or stuttering: The 9 V battery may be drained. Speakers draw sharp current pulses; if the battery voltage sags below 4.5 V, the 555 will reset erratically. Adding a 100 µF to 470 µF decoupling capacitor across the power rails can smooth this out.

Frequently Asked Questions