Siberia Racing
Tech Pages

Interlocking Track Call Push Buttons with the Track Power Relays

Steve Medanic

Track call pushbuttons are a useful way for the marshalls to turn track power off during a race.  If pushed a second time the track call pushbutton can restart the race.  If the pushbutton is held down the track call pushbutton can stop and then immediately restart the race.  This quick track call is not desired.  This article discusses installation of a track call pushbutton that is interlocked with the track power relay or relays to prevent the pushbutton from restarting the race once a track call is initiated and track power is off.  Once the track call is over the race is restarted by using the computer keyboard or mouse.

The interlock places a normally open contact from the track power relay in series with the track call pushbutton.  The interlock relay contact is separate from the contact that connects the power supply to the track.  This article discusses the basic circuit and two options.  The first option allows the pushbutton to work with Slotrak’s individual lane control function.  The second option is for those who have Trakmate hardware and use both Slotrak and Trakmate software.  The second option is installed at my track.  It allows both the Trakmate or Slotrak software to operate the track and supports the track power features of both programs.  I have individual lane power relays and can run Slotrack individual lane control and the race to the line mode without problem.  When I run Trakmate its power features also function without problem.  Here’s how it all works.

Figure 1 shows the basic Track Call Pushbutton interlock with a single power relay.  This works for any software package that uses a single power relay.   


Figure 1 – Basic Track Call Interlock


Slotrak individual lane control is performed by the use of a USB interface card.  The typical card used for Slotrak individual lane control is the 0/0/4 Phidget Interface Card.  The Phidget card provides an easy to connect interface between the computer’s USB ports and higher-voltage or current devices such as incandescent light bulbs, high-power relays, and motors  The 0/0/4 Phidget interface card contains four relay outputs for switching either AC or DC power.  Figure 2 shows the current form of this card.  The 0/0/4 designation indicates that the card has zero analog inputs, zero digital inputs and four digital outputs.  The 0/0/4 card outputs consist of four Single Pole Double Throw (SPDT) relays.  The SPDT relay contacts are rated at 10 amps AC (5 amps DC) and can be used to directly power the lanes or power interposing relays.  Most tracks use interposing relays.  The interposing relays typically have higher rated contacts (20, 30 or 40 amp).  The interposing relays also protect the Phidget or Trakmate card from damage in the event of a short.  Relay contact ratings typically apply only to resistive loads.  A typical slot car motor is an inductive load and relay contact inductive load ratings for DC loads are significantly lower than their resistive ratings.  A relay contact with a 10 amp DC resistive rating can have a low as a 1 amp DC inductive load rating.  Interposing relays are typically 12V automotive relays costing less than $2.00 each.  The 0/0/4 Phidget card retails for $60.00  In the event of a short, the inexpensive interposing relay protects the more expensive Phidget interface card from damage.  For these reasons the use of interposing relays to switch track power is recommended.

Description: C:\Users\Smedanic\Desktop\New folder (2)\1014.jpg

Figure 2 - Phidget 1014 or 0/0/4 Relay Interface Card.


Figure 3 shows the interlock used with a Phidget 0/0/4 relay card.   The Phidget card provides the individual lane control function.  The diodes allow current to flow from a Phidiget card output to the lane power relay and to the track call relay.  The diodes prevent one Phidget card output from powering more than its assigned lane.  When any single lane is powered up the common relay is energized and the track call pushbutton input is enabled.  Only when power is removed from all four lanes is the track call pushbutton disabled.   This allows one lane to be turned off and still retain the track call function for the remaining lanes.


Figure 3 – 0/0/4 Phidget Card Track Call Interlock


My installation is shown in Figure 4.  This installation adds a second set of diodes that allow power to flow to the individual lane relays from relay connected to the Trakmate card’s relay output.  This allows the Trackmate card to pick up all four lanes at once while retaining the Slotrak individual lane power control feature.  I also have a bypass switch installed that sends a “Power On” signal to the individual lane power relays outside of either software package.  This bypass switch has proven to be a handy option. 

With the Figure 4 configuration the Trakmate power control is configured normally.  The Slotrak lane power configuration settings are set for individual lane control with no master power relay installed.

Figure 4 – 0/0/4 Phidget Card Track Call Interlock & Trakmate Power Relay Interface


Figure 5 is a photograph of the installation on my current track named The Bear.  Figure 3 shows the board that holds the Phidget board, the Trakmate power relay , the Track Call pushbutton interlock relay and the required diodes.   The under table relay  board also provides filtered 12VDC power for the four 40-amp rated individual lane power relays, the infrared light bridge LEDs and the red “Track Power Off” LEDs that are built into the light bridge.  The board is easily removable for modification or repair.   The eight diodes are each rated at 1A and 100PIV.  The diodes and ferrite beads were obtained from Radio Shack.  The Bear’s “Tin Box” Trakmate interface card enclosure is also shown.  This enclosure is discussed in another “Tech Pages” article.  The Tin Box contains the Trakmate interface card and protects it from static discharge that has caused Trakmate cards to reset during a race.  This was a problem with the earlier cards.  The current cards are better protected against this issue, however, I believe it’s better to be conservative.  I have not had a card reset since the box was first installed on my previous track, Crystal Rock Raceway, during the 2001-02 season.


Figure 5 – “The Bear’s” Relay Board and Trakmate Card Enclosure


Note the extensive use of ferrite beads on both cards interface cables and the power supply input to the relay board.  The ferrite bead on the Phidget card’s USB cable allows my desktop computer to operate the Phidget card using a twenty-one (21) foot long standard USB cable.  Normally this length of cable would not work due to inductive kickback caused when the computer commands the Phidget board’s four relays to drop out.  The inductive kickback voltage surge associated with the relays dropping out can cause with the computer to lose communications with the Phidget card via the USB cable.  When this happens the software has to be reset to restore communications with the card.  The ferrite bead dampens this voltage surge allowing the computer to maintain communications with the card.  Testing determined during building of “The Bear” proved that my computer could reliably control the 4/4/0 Phidget card using a very long standard USB cable providing that a ferrite bead was installed at the card end of the USB cable.  Excessive voltage drop due to USB cable length finally caused the Phidget card to quit operating normally with approximately fifty (50) feet of standard USB cable installed.  The Phidget card continued to communicate with the computer but excessive voltage drop resulted in the card not being able to simultaneously energize (pick-up) all four relays.  Without the ferrite bead, operation with twenty-one (21) feet of USB cable was not reliable due to inductive kickback when the four relays were simultaneously de-energized at the end of a segment or in the event of a track call.  My installation passes the USB cable through the ferrite bead once.  For additional filtering, and if the opening in the bead allows, you can pass the cable through the bead multiple times in accordance with Figure 6.


Figure 6 – Passing Cable Twice Through a Ferrite Bead


Track call pushbuttons provide a useful way for the marshalls to turn track power off during a race.  This article discussed an easy way to interlock the track call pushbutton with the power relay or relays so that the pushbutton can initiate a track call but the track call can only be reset by the race director.  We have also discussed two options that allow Trakmate and Slotrak software to interface with the basic design.  These options can be used with any software that supports individual lane control.  This article also touched upon the “Tin Box” enclosure that protects the Trakmate card from static shock resets and the use of ferrite beads on USB and data cables to dampen voltage spikes that could interfere with data being transmitted to the computer during a race that can result in lap counter failure.

Back to the Tech Pages Menu Page