Model Train And Slot-Car Controller

Description: A 555 timer (U1) is configured as an astable multivibrator (oscillator) with a duty cycle of 400:1 and a frequency of 40 Hz. When power is applied to the circuit, capacitor C1 (connected to pin 6 of U1) is discharged, and the output of the 555 timer (which is used to sink current) is low. Capacitor C1 begins to charge through resistors R1 and R2 toward the positive supply rail. When the charge on C1 reaches about 66% of the supply voltage (+V), the output of U1 at pin 3 goes high. At this point, C1 begins to discharge through R2. When the charge on C1 decreases to about 33% of the supply voltage, the output of U1 returns to the low state, and the cycle is repeated until power is removed from the circuit. When the output of U1 is low, capacitor C3 is discharged into U1 via transistor Q2. When U1 pin 3 goes high, C3 charges through a current source consisting of diodes D1, D2, and resistors R3, R4, along with transistor Q1. The charge and discharge cycling of C3 produces a stream of pulses that are fed to the inverting inputs of U2A and U2B (an LM358 dual op-amp). Two voltage-divider networks (R7, R8, R9, and R10, R11, R12) set the reference voltage applied to the non-inverting inputs of U1A and U1B at pins 3 and 5. Potentiometers R9 and R12 adjust the low-level duty cycle (5 to 10%) of U1A and U1B, calibrated to ensure the train headlights glow while the motor hums only slightly. Potentiometer R3 adjusts the ramp rate of C3 for a 100% duty cycle at full throttle. A double-pole, single-throw switch (S1A and S1B) is utilized to include R3/C4 and R4/C5 in the circuit. The combinations of R5/C4 and R6/C5 result in the reference voltages presented to the non-inverting inputs of U2A and U2B changing very slowly during throttle adjustments. When the ACL/DCL switch is turned off, the resistance of the throttle-divider networks is significantly smaller than that of R5 and R6, causing the reference voltages on C4/C5 to change "instantly" to the new throttle setting. The output drivers consist of resistors R13 and R15, and transistors Q3 and Q4 for output "A," as well as resistors R17 to R20 and transistors Q5 and Q6 for output "B." Components R13/R16/Q3 and R17/R20/Q5 limit the output drive currents of Q4 and Q6 to approximately 3 A each. Resistors R14/R15 and R18/R19 activate Q4 and Q5, respectively, before the breakdown voltage is reached to prevent damage to the output drivers and dissipate energy stored in an inductive field (such as in a motor). The power supply delivers 18 V to the track, while voltage regulator U3 (a 78L09, 9-V, 100-mA voltage regulator) supplies power to the control circuits.

The circuit operates as follows: The 555 timer in astable mode generates a continuous square wave output, which alternates between high and low states. The frequency and duty cycle of this output are determined by the values of resistors R1 and R2, and capacitor C1. The rapid charging and discharging of C1 creates the oscillation needed for the timer. The output from the 555 timer drives the control circuitry, influencing the behavior of the connected components.

Capacitor C3 acts as a timing capacitor for generating additional pulses that modulate the control signals sent to the operational amplifiers U2A and U2B. The LM358 op-amps are configured to process these signals, allowing for further control of the outputs based on the duty cycles set by the potentiometers. The reference voltages established by the voltage-divider networks ensure that the op-amps function within the desired parameters, providing a stable output.

The inclusion of the double-pole switch allows for versatile control over the circuit's response to throttle adjustments, facilitating both gradual and instantaneous changes in output behavior. The output drivers are designed to handle significant current loads, ensuring that the circuit can effectively control motors and other inductive loads without risk of damage. The protective resistors and transistors are crucial for safeguarding the circuit against voltage spikes and ensuring reliable operation.

The overall design emphasizes flexibility and control, making it suitable for applications such as model trains or similar projects where precise modulation of speed and lighting is required. The combination of the 555 timer, op-amps, and output drivers creates a robust system capable of performing complex tasks while being relatively simple to implement. As shown, a 555 timer (Ul) is configured as an astable multivibrator (oscillator) with a 400:1 duty cyc le and a frequency of 40 Hz. When power is applied to the circuit, capacitor CI (connected to pin 6 of Ul) is discharged and the output of the 555 (which is used to sink current) is low. Capacitor CI begins to charge via R1 and R2 toward the positive supply rail. When the charge on CI reaches about 66% of + V, the output of Ul at pin 3 goes high. At that point, CI begins to discharge through R2. When the charge on CI decreases to about 33% of the supply voltage, the output of Ul returns to the low state, and the cycle is repeated until power is removed from the circuit.

When the output of Ul is low, C3 is discharged into Ul via transistor Q2. When Ul pin 3 goes high, C3 charges through a current source that consists of Dl, D2, R3, R4, and Ql. The charge/discharge cycling of C3 produces a stream of pulses that are fed to the inverting inputs of U2A and U2B (an LM358 dual op amp).

Two voltage-divider networks (R7, R8, R9, and RIO, Rll, R12) set the reference voltage that is applied to the noninverting inputs of U1A and U1B at pins 3 and 5. Potentiometers R9 and R12 set the low-level duty cycle (5 to 10%) of U1A and U1B. They are adjusted so that the train headlights glow, but the motor hums only slightly. Potentiometer R3 adjusts the ramp rate of C3 for 100% duty cycle at the full throttle setting. A double-pole, single-throw switch (SIA and SIB) is used to place R3/C4 and R4/C5 in the circuit. The R5/C4 and R6/C5 combinations cause the reference voltages presented to the noninverting inputs to U2A and U2B to change very slowly when the throttle is turned up and down.

When the ACL/DCL switch is turned off, the resistance of the throttle-divider networks are much smaller than those of R5 and R6, so the reference voltages on C4/C5 change "instantly" to the new throttle setting. The output drivers consist of resistors R13 and R15, and transistors Q3 and Q4 for output" "; and resistors R17 to R20, and transistors Q5 and Q6 for output "B." Components R13/R16/Q3 and R17/R20/Q5 limit the output drive currents of Q4 and Q6 to about 3 A each.

Resistors R14/R15 and R18/R19 turn on Q4 and Q5, respectively, before the breaKOhmver voltage is reached to prevent damage to the output drivers and dissipate the energy that is stored in an inductive field (such as in a motor). The power supply delivers 18 V to the track, ltage regulator U3 (a 78L09 9-V, 100-mA voltage regulator) supplies power to the control circuits.