The Belfry Bat Detector Kit - This simple ultrasonic detector was originally available in kit form. Not sure if it still is, or if you can only purchase a pre-built version. Not recommended. The quality of the printed circuit board provided was very poor. For the short time that the detector worked: It produced only simple clicks, to represent the ultrasonic calls of bats within range. The components provided are all low-cost. The unit failed on me, during a very exciting bat walk (through Central Park, NY). Not due to my assembly, by the way (I'm a certified Electronics Engineer) -but- due to a thin, cheap, circuit board trace coming loose from the board. Not only was it a huge disappointment, for one of my first bat walks, but it was also embarrassing!
In fact, I'm reluctant to even provide the link to this one, but I will - So that you know which one to avoid! Belfry detector Don't waste your time.
The Simple Detector Kit - From Tony Messina (a really nice guy! and a pillar of the North American bat hobbyist community). Now this is an entry-level detector kit that I can recommend! I really loved building these kits - I believe I've built 3 or 4, in total. Tony provides a high-quality printed circuit board that is very sturdy & well-made. A lot of thought was put in to the producing and offering of this circuit. It's neat, runs off a standard 9-volt battery (for a while!) and fits in just about any pocket. The Simple Detector Kit, is geared towards folks who have experience building simple electronic kits. There is a thorough Parts List on his site: Tony Messina's Page One would order a PCB or two, from Tony - Then place an order from your favorite electronic component supplier. Such as: DigiKey, Maplin, Mouser, etc. Only basic soldering skills and a drill are required. Listening can be done with a simple earphone; or as I mostly used mine: With a mono audio cable, leading to a "voice-activated" recorder (for un-attended monitoring of nightly bat activity).
The unit is a frequency division type detector, in this case, it divides the echolocation calls by 10.
Did I mention they were fun to assemble? ; )
The Magenta MkIIb - A nice, adjustable heterodyne bat detector. Not terribly expensive, and it's fairly sensitive. Again, un-attended monitoring of nightly bat activity is possible, by using the Headphone jack.
By using a mono audio cable, leading to a "voice-activated" recorder. The detector is a little on the large side, definitely won't fit in any normal-sized pockets. It is probably the largest, most bulky detector I've owned.
The "b" in the model name, represents the fact that it is Built. These days, this model is only available in kit form, which needs to be assembled. All parts, etc. are included: Magenta MkII
The Ciel CDB205 - This was one of my favorite bat detectors. It is an FD (Frequency Division) detector. It has very good sensitivity, and yes, it looks very cool! The two green button switches on the right-hand-side: Both the Power on, and the (white LED) "torch" buttons are internally illuminated. It is very comfortable to hold, in the hand. And, I've used it extensively, for un-attended monitoring (and recording) of nightly bat activity. Ciel electronique
I have owned/and/or built other bat detectors as well, but they are either no longer available or difficult to get the required parts for - So, I have not listed them. Now that I've posted my thoughts on all the detectors that I've previously owned & used - I can go forward with reviews of all present and future detectors.
Happy bat detecting!