Various diagram

The precision time delay circuit shown will provide accurate and repeatable time delays adjustable from a few milliseconds to a minute or two.
This is a very flexible circuit in which the operating currents and voltage depends only on the choice of SCR. The timing sequence may be initiated either by applying power to the circuit or by opening a shorting switch wired across C1. Timing capacitor C1 is charged via R1 and R2 until the voltage across C1 reaches the peak point voltage of the UJT O1. When this occurs, Q1 fires, generating a pulse across R4, triggering the SCR, and applying power to the load. Holding current for the SCR is provided via R5 and D1. The circuit is ireset by momentarily removing the supply voltage. If the circuit is to be used in an application where both rapid cycling  and accurate, repeatable timing is required - some provision must be made to ensure that C1 is discharged to zero before each-timing sequence. This can most easily be done by interconnecting a pair of switch contacts with the reset system so as to momentarily shortout C1 whenever the circuit is reset.  Temperature compensation for this circuit is provided by _R3._|increasing the value of this resistor causes the circuit to have a positive temperature coefficient. It is possible to obtain zero coefficient over a small range of ambient temperatures by optimizing » R3.

Simple UJT Repeat Timer Circuit