I tried using the Twin-T some 30 years ago and ran into problems. Back then I resolved none of them. So to start this project I tried to find as much info as I could on the Twin-T and its use.
Some History
In the June-August 1958 issues of Model Railroader magazine, Lin Westcott presented the design for a new, train block detector based on relatively new electronic components called… transistors. He called it the Twin-T because it was based on a pair of transistors (later including a third) for the basic circuit. Over the years it was updated to replace the ancient transistor antiques that Westcott used with some added modifications. Bruce Chubb, who was an early adopter of the Twin-T (MR Oct 1970 and MR May 1972), and later abandoned it for his own optimized detector design. In the August 1980 MR, Don W. Hansen wrote “The Twin-T Revisited” which offered several variations with updated transistor components. My revision is based on one of Don’s basic designs.
The recent request for help noted that the Twin-T circuit lit its indicator when the block was empty (original variation), and that needed to be reversed. Further, this need to operate multiple remote panel, block occupancy LEDs, have a local on-board, occupancy LED for testing, and also indicate when power was connected and “ON.” It also needed to be easy to build and low cost. Glossing over the multiple versions that were rejected, the Twint4 was built.
First Twint4 Design (Non Optmized)
The basic design uses a 1N5400 back to back diode pair to sense current flow to the block in both directions and turn on one of the twin pair of transistors (Q1 and Q2). A green 3mm LED is used to indicate power on, and a blue 3mm LED is used to indicate block detection. The detector operated with its own, independent 12 Volt power source. To detect the presence of a loco or car when the throttle was off, the detector injected a small current into the block via a resistor from its 12 Volt supply. So far, so good. However, the original breadboard showed some odd behavior.
My “power pack/throttle” was a “laboratory-grade” power supply. The detector “sort-of” worked. It would certainly detect the presence of a loco and allow movement forward and back. But it would not detect the loco with the power/throttle “off.” …Huh?? Undaunted, I found the last commercial DC throttle I once used, and tried it. I got slightly different results, this time with reduced speed in one direction, even though it did detect the loco now when the throttle was off. …What? …Huh?? I went back to search the literature and found 2 odd references. One was in Don’s 1980 article where he noted that with many solid state throttles the Twin-T may not detect a car in the block because the throttle voltage does not go to zero when the throttle is set to zero, and as such, cancels the bias voltage (from the DB connection on the detector). In a letter to the Clinic editor in MR October 1972 a “solution is proposed” using 4 diodes to “isolate” the power pack from the detector. I tried the 4 diode solution, and it changed but did not universally solve the problem.
Don was correct about the source of the problem. The small injected current interacted with modern regulation and momentum circuitry in later DC throttles. It really interacted with my lab power supply, the output of which is monitored by the supply, and internal circuitry is constantly active to forcibly maintain the voltage at the output under all circumstances – including overriding current from small block detectors! But wait – for decades this was virtually a standard for block detectors! I went back to my old DC throttle and turned off the momentum and braking. More change – the detector and loco behavior was a bit better. Back to the archives of ancient goodies – I found one of the first DC power packs I ever had, hooked it up and… everything worked perfectly! Oh boy, oh boy, oh boy!! But why? Well, because in the 1950’s and 1960’s the most common throttle and power pack was an unfiltered, unregulated, DC supply attached to the track via a rheostat (a variable power resistor). There was nothing to interfere with the Twin-T. The Twin-T was a good DC block detector for its time. The more electronics you inserted into loco control, the more unpredictable the results without understanding the electronics. Fortunately for my friend, his throttles of choice were all old, but solid throttles, without complicated electronics, and the Twint4 detector worked great! Yeah!!
Along the way, I also read that Twin-T detectors were incompatible with DCC layouts – wrong! Without the bias resistor (the R2 510 Ohm ½ Watt resistor DB connection) this detector works well with DCC blocks, and only requires one rail to the block to be gapped. Please see the DCC hook up diagrams below.
The output of the detector (LOAD) is an open collector output which can be connect to a logic circuit, but make sure you use a pull up resistor connected ONLY to the Arduino/logic power (Usually +5 Volts). Remember that these were originally used for panel indications of track occupancy. The basic detector presented here will have LEDs flickering from dirty track and wheels and in block crossings. There is no signal conditioning and there never was without additional circuitry.
Low Cost Block Detectors for DC and DCC
Here is the updated Twint4 design with component mods for increased sensitivity and decreased power consumption:
Below you will find:
- Details for fabricating small detector PC boards
- A step-by-step guide for building the detectors
- Detector connections to track and DC power packs/throttles
- Detector connections to track and DCC base stations/boosters
- A bill of materials (BOM) for the detectors with part examples from Digikey.com and alternate sources
You can leave off R7 and the green 3mm LED if you don’t want a “power on” indicator.
You can leave off R4, R6, Q4, and the blue 3mm LED if you don’t want an on-board block indicator.
You can substitute very low cost ($0.02 each) plastic package TO-92 2N2222A transistors for the metal TO-18 2N2222A’s, but the output current for the plastic package is 600ma max vs. 800ma for the metal TO-18. Also be very careful of the pinout order in the plastic package, they are reversed by certain manufacturers.
Metal or Plastic (Lower Cost) Transistors Can be Used
You don’t need to read schematics to use these. I designed a small PC board that anyone can order for themselves. It is a two layer board of 1.86x1.27 inches (47.27x32.16mm). Specific, step-by-step instructions for the build follow. The PC Board file named
TwinT4.brd labeled with the original Twin-T component values can be downloaded here:
http://mrhpub.com/files/users/geoffbfiles/TwinT4.zipAn updated board with updated component labels named TwinT5.brd can be downloaded here:
The PC Boards can be ordered from this board fabricator: oshpark.com.
OSHPark offers a public service fabricating very high quality, low cost PC boards in low quantities. Set up an account at OSHPark (really easy) and upload the board file you want to fabricate and specify the quantity (always in multiples of 3—their rules) unless you order the large quantity slower run boards. They accept Paypal. I have no vested interest in OSHPark – I’m just a satisfied customer. Please feel free to use whatever fabricator you know. The bare boards are shipped “panel-ized” so break or cut them apart.
New update: It looks as though Model Railroad Control Systems will be stocking the bare board as of December 2017 or so. Look for them here on the "Community Page" at
http://www.modelrailroadcontrolsystems.com/community-designs/ If any other source becomes available I'll post it here. I have no stake in
any of these sources.
I want to thank my friend Vince, for encouraging me to explore this path and solve an old problem for me, and a newer one for him. These are relatively easy to build and could easily be used with DC, DCC and AC power. If you hand wired these on perf board with plastic transistors it would be possible to build a block detector for less than $4.00 per detector. I hope some other modelers can make use of these too.
Have fun!
Best regards,
Geoff Bunza
Let’s Build Some Block Detectors
Follow the following build diagrams and solder the components to the board. Use a low temperature soldering iron or a low wattage soldering iron for these.
Original Detector and Detector with Updated Labeling
DCC Detector- Layout Connections
DCC Detector- Layout Connections
For good prices for quality surplus electronics you can use:
for transistors, diodes, LEDs, switches, relays, and many other components
for LEDs especially these high brightness 3mm green LEDs:
but for many other components too.
Ebay.com offers very low prices but be wary of some low quality parts sometimes offered.
Twin-T4 BOM | Need Per Board | Digikey Part Number | Manufacturer Part Number | Description | Part Cost | Board Cost |
1N5400 | 2 | 1N5400GOS-ND | 1N5400G | DIODE GEN PURP 50V 3A | 0.19 | 0.38 |
2N2222A | 5 | 2N2222ACS-ND | 2N2222A | TRANS NPN 40V 0.8A TO-18 | 0.40 | 2.00 |
510 Ohm 1/2 W | 1 | 510H-ND | CFR-50JB-52-510R | RES 510 OHM 1/2W 5% AXIAL | 0.10 | 0.10 |
1000 1/4W | 3 | 1.00KXBK-ND | MFR-25FBF52-1K1 | RES 1.0K OHM 1/4W 1% AXIAL | 0.10 | 0.30 |
10,000 1/4W | 3 | 10.0KXBK-ND | MFR-25FBF52-10K | RES 10K OHM 1/4W 1% AXIAL | 0.10 | 0.30 |
Blue 3mm LED | 1 | LTH3MM12VFR4600-ND | LTL1CHTBK5 | LED BLUE CLEAR 3MM ROUND 12000mcd | 0.25 | 0.25 |
Green 3mm LED | 1 | 732-5012-ND | 151033GS03000 | LED GREEN CLEAR 3MM ROUND 15000mcd | 0.25 | 0.25 |
Screw Term 5mm | 1 | 732-10959-ND | 691137710006 | CONN TERM BLOCK 5.0MM 6POS PCB | 1.27 | 1.27 |
| | | | | | |
PC Board via OSHPark.com | 1 | TwinT4.brd | High Volume Build | OSHPark.com Board Fabrication | 2.35 | 2.35 |
| | | | Single Board Cost | | 6.97 |