Picaxe itself cannot reliably switch this relay model in a capacitor design.
Excessive voltage drop on the output pin will provides for the relay only 5V * 250Ω / (250Ω + 90Ω
= 3.7V.
It is still possible to power the relay through the 74AC IC. The inverters
74AC04 have an output voltage of >4.36V or <0.36V at 24mA and Vcc = 5V.
This corresponds to 27 Ω / 15Ω in the linearized output voltage drop model. Then the relay voltage is 5V*250Ω/(250Ω+27Ω
=4.5V in the state H and 4.7V in the state L.
The voltage on the relay decreases exponentially Ure = Ureo * (1 - exp(t/T)), where T is time constant of the relay resistance (plus 74AC output) and the capacitor.
- When Uro = 4.5V and Ure = 3.75V, then t/T is approximately. (1-Ure/Ureo), of which then t/T = 0.16
- According to the datasheet, the relay needs a on/off time
t >= 10 ms. Required time constant T >= 10ms/0.16 = 62.5ms
- Since the total resistance is 250 + 27 = 277Ω and T = R * C, the capacitor should be C> T / R = 62.5ms / 277Ω =
225uF.
- When the ceramics are too expensive, so the tantalum capacitor would be suitable for long-term reliability.
When the time constant is 62.5 ms, you need to keep the relay in a new state approximately in time 3 * T = 200 ms until the capacitor is fully charged / discharged.
It is not the minimal variant, but reliable variant.
pictures:
74AC04 - selected parameters
EC2 relay - selected parameters
LatchRelay&74AC04_schema
LatchRelay&74AC04_diagram
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