Pinewood Derby Tools

Scale Construction

This page provides information and photos to aid in the construction of the Pinewood Derby Car Scale.  The focus for this page is on the hardware and circuits. Refer to the Software page for details on how to load the scale sketch (software) onto the microcontroller.

The following subjects are covered on this page:

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Parts List

The table below list the components required to build the car scale.  Refer to the schematic for the layout of the circuit. Where applicable, the ID in the table below corresponds to the component designations shown on the schematics.

Most of the components listed below can be purchased directly from this site - visit the Store for more information.
Scale Components

The following components are needed to build the scale circuit.

ID Item Description Manufacturer (Part #) Quantity
U1 Microcontroller Teensy 4.0 PJRC.com (TEENSY40)
J1 TFT Display 2.8" color 320x240 TFT display PJRC.com (DISPLAY-ILI9341)
J2, J3, J4 Load Cell Amplifier HX711 amplifier board sparkfun (SEN-13879) / Others 3
- Load Cell 100g load cell sparkfun (SEN-14727) / Others 3
- Scale PCB PCB to aid with wiring dfgtec.com 1 [o]
R1 Current limiting resistor  100 OHM resistor (axial, 1/4W, 5%) Digi-Key (100QBK-ND) 1
S1, S2 Function buttons Tactile button switch (6mm) Digi-Key (SW402-ND) 2
J6 Power jack 2.1mm DC power jack Digi-Key (CP-102AH-ND) 1
- Power supply 5V DC power supply, 500mA Digi-Key (1470-2771-ND) 1
- Power switch Power switch for scale Digi-Key (EG5676-ND) 1 [o]
- 5mm board support Supports display on scale PCB Digi-Key (732-9626-ND)
- 11mm spacer/standoff Supports scale electronics in enclosure Digi-Key (732-12704-ND) 4 [o]
- 50g calibration weight (Class M1) Used for scale calibration Various 1

Key: o=optional component, x=NOT available in the dfgtec.com Store

Microcontroller Support

Microntrollers are defined by a variety of parameters: architecture, voltage, CPU speed, number of IO, communication protocols, memory, etc. Since it would be impossible to implement the scale with every microcontroller available, only the following microcontrollers will be supported (answer questions, provide schematics, offer PCBs, etc.):

NOTE: At this time there are no plans to expand this list.

Display Support

Displays are defined by a variety of parameters: size, controller, resolution, voltage, communication protocol and they require a specific library for each microcontroller based on the controller and communication protocol. Since it's not practical to support the enormous number of possible combinations only the following displays will be supported (answer questions, supported in sketch, etc.):

Support for additional displays is planned.

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Circuit Construction

Although optional, the scale PCB simplifies the construction of the car scale by providing mounting and wiring points for the various components including the microcontroller, the three load cell amplifiers and the TFT display. This allows the larger components to be mounted directly against the PCB reducing the overall size of the scale electronics.  Most of the instructions below refer to the use of the PCB - if this is not the case for your build simply refer to the schematic for the appropriate connections between each component.

Soldering: Basic soldering is required to attach all components, headers and wires to the PCB.  If you are new to soldering here are a few suggestions:
- Search YouTube (or your favorite video site) for videos covering the basics of soldering components to PCBs.
- Start with the power jack, switches or resistor - they are the most heat tolerant.
- After soldering a component trim the excess leads (or header pins) to just above the solder joint.
Order of Construction: For the most part the order in which the components are placed on the PCB doesn't matter with one exception - the TFT display. Once the TFT display is attached to the PCB it effectively covers the mounting points of several components including the microcontroller and all three load cell amplifiers. For a successful build the TFT display MUST be installed last on the PCB.
PCB Color and Orientation

The color of the PCB may vary from the pictures on this page. The instructions below refer to the top and bottom of the PCB, here is how to tell which side is the top and which is the bottom:

NOTE! The TFT display and function switches are mounted to the top of the PCB, ALL other components are mounted to the bottom of the PCB.
Components with Headers

Standard 0.1" (2.54 mm) male headers are used to mount the microcontroller, TFT display and three load cell amplifiers to the scale PCB. If needed, the header strips can be cut to length using a utility knife or wire cutters. Two 14-pin headers are needed for the microcontroller and three 5-pin headers are needed for the load cell amplifiers. The TFT display comes with a 14-pin header already attached.

Typically the header is soldered to the component and then the component (with header attached) is soldered to the PCB. When mounted correctly the header 'body' (usually black but can be other colors) is sandwiched between the component and the PCB as shown in the picture below.

Microcontroller mounted on PCB with header
Microcontroller and Load Cell Amplifiers

The picture below shows the microcontroller and load cell amplifiers mounted on the bottom of the scale PCB. Note the orientation of each component in relation to the scale PCB.

Microcontroller and load cell amps mounted on PCB

The amps have connections on both ends, make sure the end with the Vcc and Gnd connections are attached to the PCB. Some amps only have one voltage connection and thus only 4 pins on the end connected to the PCB. In this case make sure to use the 4 left-most mounting holes on the PCB (the Gnd pin on the amp would go all the way to the left on the PCB connection).

The microcontroller (Teensy 4.0) has 5 pin holes on the end of its board (opposite of the USB connector), a header does not need to be added on that end.

Power Jack, Switch and Display Resistor

The DC power jack and resistor are mounted in the designated location on the bottom of the scale PCB as shown in the picture below. If a power switch is desired - when putting the scale in an enclosure, for example - it can be wired to the Switch connections on the bottom of PCB near the DC power jack. If a power switch will NOT be used simply jump the Switch connecters together with a short piece of wire.

DC jack, power switch and resistor mounting
Function Switches

The function switches are mounted in the designated location on the top of the scale PCB as shown in the picture below. Note that the PCB may vary from the one shown in the picture - mount a switch in the "Units" and "Tare" position on the PCB that you have.

Scale function switches
TFT Display

The TFT display is mounted on the top of scale PCB utilizing the preinstalled 14-pin header. The display will not sit square on the scale PCB because of the SD card receptacle on the back of the display. Because of this use four 5mm board spacers to support the display as shown in the first picture below. The spacers will raise the display so that the 14-pin header does not touch the scale PCB - see the second picture - this is normal.

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Load Cells

The load cells come with 4 wires pre-attached. These wires will be used to connect the load cells to the amplifiers as shown in the picture below. The wires are thin and somewhat fragile so take care when stripping them and soldering them to the amplifer. Match each colored wire to the corresponding labeled connection on the amplifier - note there is no yellow wire to connect.

Load cell wiring

Enclosure

COMING SOON! Details and plans for building an enclosure for the car scale.

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Last updated: 1 Nov 2020