The cat whisker crystal radio detector is a semiconductor used to detect radio signals. It is an old technology used in non-powered “crystal radio” sets from the first days of radio. It consists of a galena (lead sulfide) crystal and a spring-loaded wire fondly called the cat whisker. Figures 1 and 2 provide images of an antique example that was produced sometime around 1950. This device was likely marketed to young people enthusiastic to learn more about radio and electronics.
In this historical brief, we will use the modern Digilent Analog Discovery and the Digilent transistor tester to explore the operation of the antique crystal detector. We will also show the operation of the classic germanium 1N34A and a modern silicon 1N4148 diode. When combined, the historical and the modern provide a window into the operation of the small signal diodes.
Figure 1: Picture of the galena cat whisker detector being analyzed using the Digilent Analog Discovery and Digilent transistor tester.
Tech Tip: The 1N34A is becoming increasingly hard to locate. See this post for modern and readily available substitution such as the BAT46.
Figure 2: Close up image of an antique crystal radio detector. The cat whisker (spring wire) is pressing down on the galena crystal.
What is an AM radio detector?
For our conversation, the detector is the heart of an Amplitude Modulation (AM) radio. By definition, the detector detects (recovers) the audio signal that is carried on the high frequency over-the-air broadcast signal. We can think of this as a rectification process where the diode-based detector recovers the envelope of the AM broadcast signal.
A more complex analysis reveals that the detection operation, also known as heterodyne or mixing, may be carried out by nearly any non-linear electronic device. The mixing operation produces sums and difference signals between the AM carrier and the sidebands. The original audio is contained in the difference signal. The sum is removed with a filter.
However we look at it, the detector is a non-linear device. It is used to recover the audio from the high frequency AM signal.
What is meant by the term non-linear?
The linearity of an electronic device may be determined by its Ohmic response. A linear device will strictly follow Ohm’s law with a linear relationship between current and voltage with the slope determined by the device’s resistance. Given this definition, the ideal resistor is linear. Diodes and cat whiskers are non-linear.
The non-linear nature of several diodes (detectors) is shown in Figure 3. Each diode has a unique signature when plotted on this Current Voltage (IV) plot with current on the vertical axis and voltage on the horizontal. The silicon based 1N4148 does not begin to conduct until the forward bias reached about 0.5 VDC. The 1N34A and the galena cat whisker have greater sensitivity with conduction starting at about 0.2 VDC.
The non-linear action is best represented by the 1N4148. We see that no current flows until the applied voltage reaches that 0.5 VDC threshold level. Once this knee is passed, the IV relationship becomes linear reflecting the diode’s resistance.
To summarize, the diode has a complex IV relationship. There are times when it conducts and times when it doesn’t. Note that the curve in Figure 3 continues to the left. This is not shown as the ideal reverse-biased diode does not conduct.
Sensitivity
The germanium diode and the cat whisker detector have a lower threshold voltage. This might not seem significant. However, we must remember that the early crystal radio sets were unpowered. The power to drive the headphone was derived entirely from broadcast energy picked by a long-wire antenna. The 0.2 VDC conduction was an advantage that would allow the weak or distant station to be heard.
Figure 3: Representative Current Voltage (IV) curves for several diodes including 1N34A (blue), 1N4148 (red), and cat whisker (purple). Note that the germanium 1N34A and the galena detector conduct at significantly lower voltages that the silicon 1N4148.
Tech Tip: The 1N34A was prized for its operation in a crystal radio set. Like the galena cat whisker, it was responsive to small signals. Unlike the cat whisker, it did not require adjustment. While it certainly worked better, it did take some of the thrill out of the hobby.
When I was a child the 1N34A was readily available at any radio shop. Today, it is difficult to locate; at the time of this writing, the NTE product is listed as “lead time unavailable / no backorders allowed). It appears history has passed by the germanium based semiconductors in the same way as the cat whisker detector and the vacuum tube.
One potential solution for your AM crystal radio is the BAT46. Leave a comment in the space below if you would like to see the curves for the BAT46.
Varied response of the galena cat whisker detector
The galena cat whisker detector is best described as finicky. If you look back at Figure 1, you can see the little handle that the user would work to move the position of the cat whisker. It took a certain amount of skill and patience to locate the best position of the cat whisker.
A dozen different attempts are captured in Figure 4. For reference, the original 1N4148 and 1N34A curves are included. We can see that the galena detector is not consistent. Each attempt yields a different IV curve. Some of these curves may be more sensitive allowing detection of those weak AM radio stations. Other attempts would be unusable. Before departing we should recognize that the slightest bump changes the operating conditions.
Like we said, finicky.
Figure 4: This graph shows a dozen different responses from the galena cat whisker (original 1N34A and 1N4148 are shown for reference). This variability was simultaneously a source of pride, frustration, and wonderment for the early radio operator.
Parting thoughts
The crystal radio has an enduring popularity. Thousands of unique designs have been introduced over the last century; some are simple cardboard tube contraptions while others are works of art.
Modern tools such as the Digilent Analog Discovery with the added transistor tester module shed new light on the crystal heart of radio. We can see the desirable performance of the 1N34A and the potential for the galena cat whisker. The story is nicely contained within the respective IV curves.
Today, with the cell phone and ubiquity of voice-activated radio, twilight has set on the crystal radio.
Or has it?
Please share your thoughts, or better yet, post a picture of your old crystal radio that you have been keeping in a box in the attic. Maybe, you still have the aerial strung between the eaves.
Best Wishes,
APDahlen
About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (partially interwoven with military experience). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing educational articles about electronics and automation.
Highlighted Experience
Dahlen is an active contributor to the DigiKey TechForum. At the time of this writing, he has created over 150 unique posts and provided an additional 500 forum posts. Dahlen built his first crystal radio as a 4H project at the age 10 and entered it into the county fair. It featured a 1N34A diode.