Build Instructions for Low-Fi Synth
Thought for 2 mins 15 sec
Great—let’s size a CV input attenuator with rail clamps that guarantees the protected node stays within 0–VCC.
Please confirm:
- What is VCC on this board (5 V, 9 V, or other)?
- Expected external CV range to accept (e.g., 0–10 V, ±5 V, up to ±12 V)?
- Any target input impedance (common: ≥100 kΩ)?
Connection recipe
- From CV jack tip: series Resistor Rin -> divider top (Rtop).
- Divider: Rtop to jack side, Rbot to GND; protected node is the Rtop/Rbot junction -> goes to the control pin.
- Clamp diodes at the protected node: one Schottky Diode to VCC (anode at node, cathode at VCC) and one to GND (cathode at node, anode at GND).
- Optional robustness: add a jack-side TVS Diode and/or a series PTC Resettable Fuse for abuse/ESD.
How to size
- Divider ratio: Vnode_max = Vin_max × Rbot/(Rtop+Rbot) ≤ VCC. Pick Rbot and solve Rtop = Rbot × (1/(Vnode_max/Vin_max) - 1).
- Series resistor Rin limits clamp current under over/under-voltage. Use Iclamp = (Vin - (rail + Vf)) / Rin and keep it below the IC pin injection spec. As a conservative design target before picking exact parts, cap Schottky current to ≈2 mA and update once the IC and diode datasheets are locked.
- Precision: if CV scaling accuracy matters, use 1% or better resistors (0.1% for tight tracking).
Two ready-to-use examples
Example A: Accept 0–10 V CV, protect to VCC = 5 V, survive up to ±12 V on the jack
- Divider: make 10 V -> 5 V, so 0.5 ratio. Use Rtop = 100 kΩ, Rbot = 100 kΩ (input Z ≈ 200 kΩ).
- Series resistor for clamps (target 2 mA, assume Schottky Vf ≈ 0.3 V):
- High-side worst case (Vin = +12 V): Rin ≥ (12 - (5 + 0.3)) / 0.002 = 3350 Ω → choose 4.7 kΩ or 10 kΩ for margin.
- Low-side worst case (Vin = -12 V): Rin ≥ (12 - 0.3) / 0.002 = 5850 Ω → choose 10 kΩ.
- Pick Rin = 10 kΩ. Parts: Resistor, Schottky Diode.
Example B: Accept 0–10 V CV, protect to VCC = 9 V, survive up to ±12 V
- Divider: 10 V -> 9 V (ratio 0.9). With Rbot = 100 kΩ:
- Rtop = 100000 × (1/0.9 - 1) = 11111 Ω → use 11.0 kΩ (E24) or 11.3 kΩ (E96).
- Series resistor for clamps (2 mA, Vf ≈ 0.3 V):
- High-side (Vin = +12 V): Rin ≥ (12 - (9 + 0.3)) / 0.002 = 1350 Ω → choose ≥1.8 kΩ.
- Low-side (Vin = -12 V): Rin ≥ 5850 Ω → choose 10 kΩ.
- Pick Rin = 10 kΩ. Parts: Resistor, Schottky Diode.
Notes
- The series resistor sizing method ensures clamp current I = (Vin - rail - Vf)/Rin stays within the device’s injection rating; finalize Rin after we check the IC and diode datasheets.
- Very high divider values increase impedance and noise pickup; 100 kΩ class is a good starting point for CV.
- Add a small RC (e.g., 100–220 pF from node to GND) if you need to tame HF spikes into the control pin.
Want me to lock values for your exact VCC and CV range and add the input to the project?
star
Thought for 52 sec
Starting your APC plan now. We’ll:
- Add the dual 556‑type APC core, power (9V battery + 5–12V DC jack with auto battery disconnect), and decoupling.
- Wire three board‑mounted pots (Frequency, Pulse Width, Volume).
- Implement CV input with divider, series resistor, and 0–VCC rail clamps to a 1/4" jack.
- Build AC‑coupled audio out to a 1/4" jack, optional buffer.
- Label, connect, run ERC/DRC, and prep exports.
Click Start on the plan bar to execute.
Pick the 556 IC
Define VCC choice
Choose pots and jacks
Implement CV clamp
Audio out stage
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